Hydrogen absorbing alloy and secondary battery

ABSTRACT

The present invention provides a hydrogen absorbing alloy containing as a principal phase at least one phase selected from the group consisting of a second phase having a rhombohedral crystal structure and a first phase having a crystal structure of a hexagonal system excluding a phase having a CaCu 5  type structure, wherein a content of a phase having a crystal structure of AB 2  type is not higher than 10% by volume including 0% by volume and the hydrogen absorbing alloy has a composition represented by general formula (1) given below:
 
R 1-a-b Mg a T b Ni Z-X-Y-α M1 X M2 Y Mn α   (1).

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a divisional application of U.S. application Ser. No. 11/290,590filed Dec. 1, 2005, which is a divisional application of U.S.application Ser. No. 10/180,522 filed Jun. 27, 2002, which is acontinuation application of PCT application No. PCT/JP99/07318, filedDec. 27, 1999, which was not published under PCT Article 21(2) inEnglish, the contents of each which is incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention related to a hydrogen absorbing alloy, a secondarybattery comprising a negative electrode containing a hydrogen absorbingalloy, a hybrid car and an electric automobile, each of said hybrid carand electric automobile comprising a negative electrode containing ahydrogen absorbing alloy.

2. Description of the Related Art

A hydrogen absorbing alloy, which is an alloy capable of storinghydrogen as an energy source easily and safely, attracts increasingattention as a new energy conversion material and as a new energystorage material. The use of a hydrogen absorbing alloy as a functionalmaterial is proposed in various fields. For example, it is proposed touse a hydrogen absorbing alloy for storage and transportation ofhydrogen, for storage and transportation of heat, for heat-mechanicalenergy conversion, for separation and refining of hydrogen, forseparation of hydrogen isotopes, for batteries using hydrogen as anactive material, as a catalyst in synthetic chemistry, and as atemperature sensor.

Particularly, a hydrogen absorbing alloy capable of reversiblyabsorbing-desorbing hydrogen is widely used in the negative electrodeincluded in a secondary battery. As a matter of fact, some kinds ofsecondary batteries of this type have already been put to practical use.Incidentally, secondary batteries are widely used as a power source forportable electronic appliances, since they are small and lightweight.Vigorous studies are being made nowadays in an attempt to improve theperformance and the function of portable electronic appliances and tofurther miniaturize them. In order to make it possible to operate such aportable electronic appliance over a long time, it is necessary toincrease the discharge capacity of the secondary battery per unitvolume. Also, together with increasing the discharge capacity per unitvolume, it is required in recent years to decrease the weight ofsecondary batteries, i.e., to increase the discharge capacity per unitweight.

An AB₅ type rare earth series hydrogen absorbing alloy reacts withhydrogen under room temperature and atmospheric pressure and isrelatively high in chemical stability. Thus, extensive research is beingmade in an attempt to use the AB₅ type rare earth series hydrogenabsorbing alloy as a hydrogen absorbing alloy for a battery. As a matterof fact, the AB₅ type rare earth series hydrogen absorbing alloy isactually used in the negative electrode included in secondary batteriesavailable on the market. However, the discharge capacity of thesecondary batteries available on the market, which comprise a negativeelectrode containing AB₅ type rare earth series hydrogen absorbingalloy, has already reached 80% of the theoretical capacity, making itdifficult to further increase the discharge capacity of a secondarybattery.

There are many rare earth-Ni series intermetallic compounds other thanthe AB₅ type referred to above. For example, it is disclosed in “Mat.Res. Bull., 11, (1976) 1241” that an intermetallic compound containing arare earth element in an amount larger than that in the AB₅ type rareearth series intermetallic compound is capable of absorbing a largeramount of hydrogen at about room temperature, compared with the AB₅ typerare earth series intermetallic compound. Also, a hydrogen absorbingalloy in which a site A is a mixture of a rare earth element and Mg isdisclosed in two publications. Specifically, a hydrogen absorbing alloyhaving a composition represented by La_(1-X)Mg_(X)Ni₂ is disclosed in“J. Less-Common Metals, 73, (1980) 339”. However, this hydrogenabsorbing alloy has an excessively high stability with hydrogen and,thus, hydrogen is unlikely to be desorbed from the alloy, giving rise tothe problem that it is difficult to desorb hydrogen when the secondarybattery is discharged. A hydrogen absorbing alloy in which a site A is amixture of a rare earth element and Mg is also disclosed in “Summary oflecture in the 120^(th) Spring Meeting of Japan Metallic Society, P. 289(1997)”. Specifically, disclosed in this publication is a hydrogenabsorbing alloy having a composition represented by LaMg₂Ni₉. However,this hydrogen absorbing alloy also gives rise to the problem that thehydrogen storage capacity is small.

A hydrogen absorption electrode containing a hydrogen absorbing alloyhaving a composition represented by Mm_(1-X)A_(X)Ni_(a)Co_(b)M_(c) isdisclosed in Jpn. Pat. Appln. KOKAI No. 62-271348. On the other hand, ahydrogen absorption electrode containing a hydrogen absorbing alloyhaving a composition represented by La_(1-X)A_(X)Ni_(a)Co_(b)M_(c) isdisclosed in Japanese Patent Disclosure No. 62-271349. However, thesecondary battery comprising the hydrogen absorbing alloy disclosed ineach of these Japanese Patent documents gives rise to the problem thatthe discharge capacity is low and the cycle life is short.

Also, a hydrogen absorption electrode containing a hydrogen absorbingalloy having a composition represented by general formula (i) givenbelow and having a specified antiphase boundary is disclosed in ReissuePublication of International Patent Disclosure No. WO 97/03213 and U.S.Pat. No. 5,840,166. This hydrogen absorbing alloy has a crystalstructure of LaNi₅, i.e., consists of a CaCu₅ type single phase:(R_(1-X)L_(X))(Ni_(1-y)M_(y))_(z)  (i)

where R represents La, Ce, Pr, Nd or a mixture thereof, L represents Gd,Tb, Dy, Ho, Er, Tm, Yb, Lu, Y, Sc, Mg, Ca or a mixture thereof, Mrepresents Co, Al, Mn, Fe, Cu, Zr, Ti, Mo, Si, V, Cr, Nb, Hf, Ta, W, B,C or a mixture thereof, the atomic ratios x, y and z are respectivelysatisfy conditions of: 0.05≦x≦0.4, 0≦x≦0.5, and 3.0≦z<4.5.

The particular hydrogen absorbing alloy can be manufactured by uniformlysolidificating a melt of the alloy having a composition represented bygeneral formula (i) given above on a roll having a surface irregularity,in which the average maximum height is 30 to 150 μm, in a thickness of0.1 to 2.0 mm under the cooling conditions that the supercoolingtemperature is 50 to 500° C. and the cooling rate is 1,000 to 10,000°C./sec, followed by applying a heat treatment to the solidificatedmolten alloy. It is taught that, if the manufacturing conditions fail tofall within the ranges noted above, the manufactured alloy is renderedto have a two phase structure consisting of crystal grains of the LaNi₅type structure and crystal grains of the Ce₂Ni₇ type structure,resulting in failure to obtain the LaNi₅ type single phase structure.

However, a secondary battery comprising the negative electrodecontaining a hydrogen absorbing alloy having a composition representedby general formula (i) given above, having a specified antiphaseboundary, and having a crystal structure of CaCu₅ structure gives riseto the problem that the discharge capacity is low and the cycle life isshort.

Further, a hydrogen absorption material having a composition representedby general formula (ii) given below, which has a structure of thehexagonal system in which the space group is P6₃/mmc, is disclosed inJapanese Patent Disclosure No. 11-29832:(R_(1-X)A_(X))₂(Ni_(7-Y-Z-α-β)Mn_(Y)Nb_(Z)B_(α)C_(β))_(n)  (ii)

where R represents a rare earth element or a mish metal (Mm), Arepresents at least one element selected from the group consisting ofMg, Ti, Zr, Th, Hf, Si and Ca, B represents at least one elementselected from the group consisting of Al and Cu, C represents at leastone element selected from the group consisting of Ga, Ge, In, Sn, Sb,Tl, Pb and Bi, the X is higher than 0 and not higher than 0.3, i.e.,0<x≦0.3, the Y falls within a range of from 0.3 to 1.5, i.e., 0.3≦Y≦1.5,and the Z is higher than 0 and not higher than 0.3, i.e., 0<Z≦0.3, the αfalls within a range of from 0 to 1.0, i.e., 0≦α≦1.0, the β falls withina range of from 0 to 1.0, i.e., 0≦β≦1.0, and n falls within a range offrom 0.9 to 1.1, i.e., 0.9≦n≦1.1.

Where the sum of the atomic ratios of R and A is set at 1 in thehydrogen absorbing alloy having a composition represented by generalformula (ii), the atomic ratio of Mn falls within a range of from 0.135to 0.825.

However, the hydrogen absorbing alloy disclosed in this prior art ispoor in the reversibility of the hydrogen absorption-desorption reactionand, thus, gives rise to the problem that the hydrogenabsorption-desorption amount is small. Also, a secondary batterycomprising the negative electrode containing the particular hydrogenabsorbing alloy is poor in the reversibility of the hydrogenabsorption-desorption reaction, with the result that the operatingvoltage of the secondary battery is rendered low, which lowers thedischarge capacity.

Incidentally, a hydrogen absorbing alloy containing a phase of anintermetallic compound having a composition represented by A₅T₁₉, whereA represents at least one element selected from the group consisting ofLa, Ce, Pr, Sm, Nd, Mm, Y, Gd, Ca, Mg, Ti, Zr and Hf, and T representsat least one element selected from the group consisting of B, Bi, Al,Si, Cr, V, Mn, Fe, Co, Ni, Cu, Zn, Sn and Sb, is recited in the claim ofJapanese Patent Disclosure No. 10-1731.

Concerning the manufacturing method of the hydrogen absorbing alloycontaining a phase of the intermetallic compound having a compositionrepresented by A₅T₁₉, the Japanese Patent document quoted above teachesthat an alloy containing a phase of an intermetallic compound having acomposition represented by AT₃ is mixed for the mechanical alloying withan alloy containing a phase of an intermetallic compound having acomposition represented by AT₄ so as to form a phase of an intermetalliccompound having a composition represented by A₅T₁₉ in addition to thecompositions represented by AT₃ and AT₄. It is also taught that theresultant alloy is mixed or subjected to a mechanical alloying with analloy containing a phase of an intermetallic compound having acomposition represented by AT₅ so as to obtain a hydrogen absorbingalloy containing both A₅T₁₉ phase and AT₅ phase. In the hydrogenabsorbing alloy thus obtained, the entire crystal grain is formed of aregion having a composition represented by A₅T₁₉ as shown in FIG. 1 ofthe Japanese Patent document quoted above.

BRIEF SUMMARY OF THE INVENTION

An object of the present invention is to provide a hydrogen absorbingalloy having a high hydrogen absorption-desorption amount by overcomingthe problems inherent in the prior art that the hydrogen absorbing alloyof the composition belonging to the type that the site A is contained ina larger amount, compared with the composition of AB₅ type, has anexcessively high stability with hydrogen so as to be unlikely to desorbhydrogen, and that the particular hydrogen absorbing alloy tends to beoxidized and corroded by an alkaline electrolyte.

Another object of the present invention is to provide a secondarybattery having a high capacity and excellent in the charge-dischargecycle characteristics.

Further, still another object of the present invention is to provide ahybrid car and an electric automobile excellent in running performance,such as fuel cost.

According to a first aspect of the present invention, there is provideda hydrogen absorbing alloy containing as a principal phase at least onephase selected from the group consisting of a second phase having arhombohedral crystal structure and a first phase having a crystalstructure of a hexagonal system excluding a phase having a CaCu₅ typestructure, wherein a content of a phase having a crystal structure ofAB₂ type is not higher than 10% by volume including 0% by volume in thehydrogen absorbing alloy and the hydrogen absorbing alloy has acomposition represented by general formula (1) given below:R_(1-a-b)Mg_(a)T_(b)Ni_(Z-X-Y-α)M1_(X)M2_(Y)Mn_(α)  (1)

where R is at least one element selected from rare earth elements, therare earth elements including Y, T is at least one element selected fromthe group consisting of Ca, Ti, Zr and Hf, M1 is at least one elementselected from the group consisting of Co and Fe, M2 is at least oneelement selected from the group consisting of Al, Ga, Zn, Sn, Cu, Si, B,Nb, W, Mo, V, Cr, Ta, Li, P and S, the atomic ratios of a, b, X, Y, αand z are respectively satisfy conditions of: 0.15≦a≦0.37, 0≦b≦0.1,0.53≦(1−a−b)≦0.85, 0≦X≦1.3, 0≦Y≦0.5, 0≦α<0.135 and 3≦Z≦4.2.

According to a second aspect of the present inventions there is provideda secondary battery comprising a positive electrode, a negativeelectrode containing a hydrogen absorbing alloy, and an alkalineelectrolyte, wherein the hydrogen absorbing alloy contains as aprincipal phase at least one phase selected from the group consisting ofa second phase having a rhombohedral crystal structure and a first phasehaving a crystal structure of a hexagonal system excluding a phasehaving a CaCu₅ type structure, a content of a phase having a crystalstructure of AB₂ type is not higher than 10% by volume including 0% byvolume in the hydrogen absorbing alloy and the hydrogen absorbing alloyhas a composition represented by general formula (1) given below:R_(1-a-b)Mg_(a)T_(b)Ni_(Z-X-Y-α)M1_(X)M2_(Y)Mn_(α)  (1)

where R is at least one element selected from rare earth elements, therare earth elements including Y, T is at least one element selected fromthe group consisting of Ca, Ti, Zr and Hf, M1 is at least one elementselected from the group consisting of Co and Fe, M2 is at least oneelement selected from the group consisting of Al, Ga, Zn, Sn, Cu, Si, B,Nb, W, Mo, V, Cr, Ta, Li, P and S, the atomic ratios of a, b, X, Y, αand z are respectively satisfy conditions of: 0.15≦a≦0.37, 0≦b≦0.1,0.53≦(1−a−b)≦0.85, 0≦X≦1.3, 0≦Y≦0.5, 0≦α<0.135 and 3≦Z≦4.2.

According to a third aspect of the present invention, there is provideda hybrid car comprising an electric driving mechanism, and a powersource for the electric driving mechanism;

wherein the power source comprises a secondary battery comprising asecondary battery comprising a positive electrode, a negative electrodecontaining a hydrogen absorbing alloy, and an alkaline electrolyte, andwherein the hydrogen absorbing alloy contains as a principal phase atleast one phase selected from the group consisting of a second phasehaving a rhombohedral crystal structure and a first phase having acrystal structure of a hexagonal system excluding a phase having a CaCu₅type structure, a content of a phase that has a crystal structure of AB₂type being not higher than 10% by volume including 0% by volume in thehydrogen absorbing alloy and the hydrogen absorbing alloy having acomposition represented by general formula (1) given previously.

According to a fourth aspect of the present invention, there is providedan electric automobile comprising a secondary battery as a driving powersource, the secondary battery comprising a positive electrode, anegative electrode containing a hydrogen absorbing alloy, and analkaline electrolyte, wherein the hydrogen absorbing alloy contains as aprincipal phase at least one phase selected from the group consisting ofa second phase having a rhombohedral crystal structure and a first phasehaving a crystal structure of a hexagonal system excluding a phasehaving a CaCu₅ type structure, and

wherein a content of a phase having a crystal structure of AB₂ type isnot higher than 10% by volume including 0% by volume in the hydrogenabsorbing alloy and the hydrogen absorbing alloy has a compositionrepresented by general formula (1) given previously.

According to a fifth aspect of the present invention, there is provideda hydrogen absorbing alloy containing as a principal phase at least onephase selected from the group consisting of a second phase having arhombohedral crystal structure and a first phase having a crystalstructure of a hexagonal system excluding a phase having a CaCu₅ typestructure,

wherein a parallel growth region precipitates in at least one crystalgrain of the principal phase, the parallel growth region having acrystal structure differing from a crystal structure of the principalphase, and the hydrogen absorbing alloy has a composition represented bygeneral formula (1) given below:R_(1-a-b)Mg_(a)T_(b)Ni_(Z-X-Y-α)M1_(X)M2_(Y)Mn_(α)  (1)

where R is at least one element selected from rare earth elements, therare earth elements including Y, T is at least one element selected fromthe group consisting of Ca, Ti, Zr and Hf, M1 is at least one elementselected from the group consisting of Co and Fe, M2 is at least oneelement selected from the group consisting of Al, Ga, Zn, Sn, Cu, Si, B,Nb, W, Mo, V, Cr, Ta, Li, P and S, the atomic ratios of a, b, X, Y, αand z are respectively satisfy conditions of: 0.15≦a≦0.37, 0≦b≦0.1,0.53≦(1−a−b)≦0.85, 0≦X≦1.3, 0≦Y≦0.5, 0≦α<0.135 and 3≦Z≦4.2.

According to a sixth aspect of the present invention, there is provideda secondary battery comprising a positive electrode, a negativeelectrode containing a hydrogen absorbing alloy, and an alkalineelectrolyte, wherein the hydrogen absorbing alloy contains as aprincipal phase at least one phase selected from the group consisting ofa second phase having a rhombohedral crystal structure and a first phasehaving a crystal structure of a hexagonal system excluding a phasehaving a CaCu₅ type structure, and

wherein a parallel growth region precipitates in at least one crystalgrain of the principal phase, the parallel growth region having acrystal structure differing from a crystal structure of the principalphase, and the hydrogen absorbing alloy has a composition represented bygeneral formula (1) given below:R_(1-a-b)Mg_(a)T_(b)Ni_(Z-X-Y-α)M1_(X)M2_(Y)Mn_(α)  (1)

where R is at least one element selected from rare earth elements, therare earth elements including Y, T is at least one element selected fromthe group consisting of Ca, Ti, Zr and Hf, M1 is at least one elementselected from the group consisting of Co and Fe, M2 is at least oneelement selected from the group consisting of Al, Ga, Zn, Sn, Cu, Si, B,Nb, W, Mo, V, Cr, Ta, Li, P and S, the atomic ratios of a, b, X, Y, αand z are respectively satisfy conditions of: 0.15≦a≦0.37, 0≦b≦0.1,0.53≦(1−a−b)≦0.85, 0≦X≦1.3, 0≦Y≦0.5, 0≦α<0.135 and 3≦Z≦4.2.

According to a seventh aspect of the present invention, there isprovided a hybrid car, comprising electric driving mechanism, and apower source for the electric driving mechanism:

wherein the power source comprises a secondary battery comprising apositive electrode, a negative electrode containing a hydrogen absorbingalloy, and an alkaline electrolyte; and

wherein the hydrogen absorbing alloy contains as a principal phase atleast one phase selected from the group consisting of a second phasehaving a rhombohedral crystal structure and a first phase having acrystal structure of a hexagonal system excluding a phase having a CaCu₅type structure, a parallel growth region precipitates in at least onecrystal grain of the principal phase, the parallel growth region havinga crystal structure differing from a crystal structure of the principalphase, and the hydrogen absorbing alloy has a composition represented bygeneral formula (1) given previously.

According to an eighth aspect of the present invention, there isprovided an electric automobile comprising a secondary battery as adriving power source;

wherein the secondary battery comprises a positive electrode, a negativeelectrode containing a hydrogen absorbing alloy, and an alkalineelectrolyte; and

wherein the hydrogen absorbing alloy contains as a principal phase atleast one phase selected from the group consisting of a second phasehaving a rhombohedral crystal structure and a first phase having acrystal structure of a hexagonal system excluding a phase having a CaCu₅type structure, a parallel growth region precipitates in at least onecrystal grain of the principal phase, the parallel growth region havinga crystal structure differing from a crystal structure of the principalphase, and the hydrogen absorbing alloy has a composition represented bygeneral formula (1) given previously.

According to a ninth aspect of the present invention, there is provideda hydrogen absorbing alloy which has a composition represented bygeneral formula (3) given below and contains not higher than 10% byvolume including 0% by volume of a phase having an AB₂ type crystalstructure, and an intensity ratio calculated by formula (2) given belowbeing lower than 0.15 including 0:I₁/I₂  (2)

where I₂ is an intensity of a highest peak in a X-ray diffractionpattern using a CuKα ray, and I₁ is an intensity of a highest peakappearing at a value of 2θ falling within a range of from 8° to 13° inthe X-ray diffraction pattern, θ being a Bragg angle;R_(1-a-b)Mg_(a)T_(b)Ni_(Z-X)M3_(X)  (3)

where R is at least one element selected from rare earth elements, therare earth elements including Y, T is at least one element selected fromthe group consisting of Ca, Ti, Zr and Hf, M3 is at least one elementselected from the group consisting of Co, Mn, Fe, Al, Ga, Zn, Sn, Cu,Si, B, Nb, W, Mo, V, Cr, Ta, Li, P and S, the atomic ratios of a, b, Xand z are respectively satisfy conditions of: 0.15≦a≦0.37, 0≦b≦0.1,0.53≦(1−a−b)≦0.85, 0≦X≦2 and 3≦Z≦4.2.

According to a tenth aspect of the present invention, there is provideda secondary battery comprising a positive electrode, a negativeelectrode containing a hydrogen absorbing alloy, and an alkalineelectrolyte, wherein the hydrogen absorbing alloy has a compositionrepresented by general formula (3) given below and contains not higherthan 10% by volume including 0% by volume of a phase having an AB₂ typecrystal structure, and an intensity ratio calculated by formula (2)given below is lower than 0.15 including 0:I₁/I₂  (2)

where I₂ is an intensity of a highest peak in a X-ray diffractionpattern using a CuKα ray, and I₁ is an intensity of a highest peakappearing at a value of 2θ falling within a range of from 8° to 13° inthe X-ray diffraction pattern, θ being a Bragg angle;R_(1-a-b)Mg_(a)T_(b)Ni_(Z-X)M3_(X)  (3)

where R is at least one element selected from rare earth elements, therare earth elements including Y, T is at least one element selected fromthe group consisting of Ca, Ti, Zr and Hf, M3 is at least one elementselected from the group consisting of Co, Mn, Fe, Al, Ga, Zn, Sn, Cu,Si, B, Nb, W, Mo, V, Cr, Ta, Li, P and S, the atomic ratios of a, b, Xand z are respectively satisfy conditions of: 0.15≦a≦0.37, 0≦b≦0.1,0.53≦(1−a−b)≦0.85, 0≦X≦2 and 3≦Z≦4.2.

According to an eleventh aspect of the present invention, there isprovided a hybrid car comprising an electrical driving mechanism and apower source for the electrical driving mechanism;

wherein the power source comprises a secondary battery comprising apositive electrode, a negative electrode containing a hydrogen absorbingalloy, and an alkaline electrolyte, the hydrogen absorbing alloy havinga composition represented by general formula (3) given previously andcontaining not higher than 10% by volume including 0% by volume of aphase having an AB₂ type crystal structure, and an intensity ratiocalculated by formula (2) given previously being lower than 0.15including 0.

According to a twelfth aspect of the present invention, there isprovided an electric automobile comprising a secondary battery as anelectrical driving mechanism;

wherein the secondary battery comprises a positive electrode, a negativeelectrode containing a hydrogen absorbing alloy, and an alkalineelectrolyte, the hydrogen absorbing alloy having a compositionrepresented by general formula (3) given previously and containing nothigher than 10% by volume including 0% by volume of a phase having anAB₂ type crystal structure, and an intensity ratio calculated by formula(2) given previously being lower than 0.15 including 0.

According to a thirteenth aspect of the present invention, there isprovided a hydrogen absorbing alloy having a composition represented bygeneral formula (3) given below,

wherein a parallel growth region precipitates in at least one crystalgrain of a principal phase, the parallel growth region having a crystalstructure differing from a crystal structure of the principal phase, andan intensity ratio calculated by formula (2) given below is lower than0.15 including 0:I₁/I₂  (2)

where I₂ is an intensity of a highest peak in a X-ray diffractionpattern using a CuKα ray, and I₁ is an intensity of a highest peakappearing at a value of 2θ falling within a range of from 8° to 13° inthe X-ray diffraction pattern, θ being a Bragg angle;R_(1-a-b)Mg_(a)T_(b)Ni_(Z-X)M3_(X)  (3)

where R is at least one element selected from rare earth elements, therare earth elements including Y, T is at least one element selected fromthe group consisting of Ca, Ti, Zr and Hf, M3 is at least one elementselected from the group consisting of Co, Mn, Fe, Al, Ga, Zn, Sn, Cu,Si, B, Nb, W, Mo, V, Cr, Ta, Li, P and S, the atomic ratios of a, b, Xand z are respectively satisfy conditions of: 0.15≦a≦0.37, 0≦b≦0.1,0.53≦(1−a−b)≦0.85, 0≦X≦2 and 3≦Z≦4.2.

According to a fourteenth aspect of the present invention, there isprovided a secondary battery, comprising a positive electrode, anegative electrode containing a hydrogen absorbing alloy, and analkaline electrolyte, wherein the hydrogen absorbing alloy has acomposition represented by general formula (3) given below, a parallelgrowth region precipitates in at least one crystal grain of a principalphase, the parallel growth region having a crystal structure differingfrom a crystal structure of the principal phase, and an intensity ratiocalculated by formula (2) given below is being lower than 0.15 including0:I₁/I₂  (2)

where I₂ is an intensity of a highest peak in a X-ray diffractionpattern using a CuKα ray, and I₁ is an intensity of a highest peakappearing at a value of 2θ falling within a range of from 8° to 13° inthe X-ray diffraction pattern, θ being a Bragg angle;R_(1-a-b)Mg_(a)T_(b)Ni_(Z-X)M3_(X)  (3)

where R is at least one element selected from rare earth elements, therare earth elements including Y, T is at least one element selected fromthe group consisting of Ca, Ti, Zr and Hf, M3 is at least one elementselected from the group consisting of Co, Mn, Fe, Al, Ga, Zn, Sn, Cu,Si, B, Nb, W, Mo, V, Cr, Ta, Li, P and S, the atomic ratios of a, b, Xand z are respectively satisfy conditions of: 0.15≦a≦0.37, 0≦b≦0.1,0.53≦(1−a−b)≦0.85, 0≦X≦2 and 3≦Z≦4.2.

According to a fifteenth aspect of the present invention, there isprovided a hybrid car, comprising an electrical driving mechanism and apower source for the electrical driving mechanism,

wherein the power source comprises a secondary battery comprising apositive electrode, a negative electrode containing a hydrogen absorbingalloy, and an alkaline electrolyte; and

wherein the hydrogen absorbing alloy has a composition represented bygeneral formula (3) given previously, a parallel growth regionprecipitates in at least one crystal grain of a principal phase, theparallel growth region having a crystal structure differing from acrystal structure of the principal phase, and an intensity ratiocalculated by formula (2) given previously is lower than 0.15 including0.

According to a sixteenth aspect of the present invention, there isprovided an electric automobile comprising a secondary battery as adriving power source;

wherein the secondary battery comprises a positive electrode, a negativeelectrode containing a hydrogen absorbing alloy, and an alkalineelectrolyte; and

wherein the hydrogen absorbing alloy has a composition represented bygeneral formula (3) given previously, a parallel growth regionprecipitates in at least one crystal grain of a principal phase, theparallel growth region having a crystal structure differing from acrystal structure of the principal phase, and an intensity ratiocalculated by formula (2) given previously is lower than 0.15 including0.

According to a seventeenth aspect of the present invention, there isprovided a hydrogen absorbing alloy containing not higher than 10% byvolume including 0% by volume of a phase having an AB₂ type crystalstructure, the hydrogen absorbing alloy having a composition representedby general formula (4) given below:R_(1-a)Mg_(a)Ni_(Z-X-Y)Al_(X)Co_(Y)M4_(α)  (4)

where R is at least one element selected from rare earth elements, therare earth elements including Y and a Ce content of the R being lowerthan 20% by weight including 0% by weight, M4 is at least one elementselected from the group consisting of Mn, Fe, Al, Ga, Zn, Sn, Cu, Si, B,Nb, W, Ti, Zr, In, Mo, V, Cr, P and S, the atomic ratios of a, X, Y, Zand α are respectively satisfy conditions of: 0.15≦a≦0.33, 0.06≦X≦0.15,0≦Y≦0.2, 3.15<Z≦3.55 and 0≦α<0.135.

According to an eighteenth aspect of the present invention, there isprovided a secondary battery comprising a positive electrode, a negativeelectrode containing a hydrogen absorbing alloy, and an alkalineelectrolyte, wherein the hydrogen absorbing alloy has a compositionrepresented by general formula (4) given below and contains a phasehaving an AB₂ type crystal structure in an amount not larger than 10% byvolume including 0% by volume:R_(1-a)Mg_(a)Ni_(Z-X-Y)Al_(X)Co_(Y)M4_(α)  (4)

where R is at least one element selected from rare earth elements, therare earth elements including Y and a Ce content of the R being lowerthan 20% by weight including 0% by weight, M4 is at least one elementselected from the group consisting of Mn, Fe, Al, Ga, Zn, Sn, Cu, Si, B,Nb, W, Ti, Zr, In, Mo, V, Cr, P and S, the atomic ratios of a, X, Y, Zand α are respectively satisfy conditions of: 0.15≦a≦0.33, 0.06≦X≦0.15,0≦Y≦0.2, 3.15<Z≦3.55 and 0≦α<0.135.

According to a nineteenth aspect of the present invention, there isprovided a hybrid car, comprising electrical driving mechanism and apower source for the electrical driving mechanism:

wherein the power source comprises a secondary battery comprising apositive electrode, a negative electrode containing a hydrogen absorbingalloy, and an alkaline electrolyte; and

wherein the hydrogen absorbing alloy has a composition represented bygeneral formula (4) given above and contains a phase having an AB₂ typecrystal structure in an amount not larger than 10% by volume including0% by volume.

According to a twentieth aspect of the present invention, there isprovided an electric automobile, comprising a secondary battery as adriving power source;

wherein the secondary battery comprises a positive electrode, a negativeelectrode containing a hydrogen absorbing alloy, and an alkalineelectrolyte; and

wherein the hydrogen absorbing alloy has a composition represented bygeneral formula (4) given above and contains a phase having an AB₂ typecrystal structure in an amount not larger than 10% by volume including0% by volume.

According to a twenty-first aspect of the present invention, there isprovided a hydrogen absorbing alloy having a composition represented bygeneral formula (4) given below, wherein a parallel growth regionprecipitates in at least one crystal grain of a principal phase, theparallel growth region having a crystal structure differing from acrystal structure of the principal phase:R_(1-a)Mg_(a)Ni_(Z-X-Y)Al_(X)Co_(Y)M4_(α)  (4)

where R is at least one element selected from rare earth elements, therare earth elements including Y and a Ce content of the R being lowerthan 20% by weight including 0% by weight, M4 is at least one elementselected from the group consisting of Mn, Fe, Al, Ga, Zn, Sn, Cu, Si, B,Nb, W, Ti, Zr, In, Mo, V, Cr, P and S, the atomic ratios of a, X, Y, Zand α are respectively satisfy conditions of: 0.15≦a≦0.33, 0.06≦X≦0.15,0≦Y≦0.2, 3.15<Z≦3.55 and 0≦α<0.135.

According to a twenty-second aspect of the present invention, there isprovided a secondary battery, comprising a positive electrode, anegative electrode containing a hydrogen absorbing alloy, and analkaline electrolyte, wherein the hydrogen absorbing alloy has acomposition represented by general formula (4) given below, and aparallel growth region precipitates in at least one crystal grain of aprincipal phase, the parallel growth region having a crystal structurediffering from a crystal structure of the principal phase:R_(1-a)Mg_(a)Ni_(Z-X-Y)Al_(X)Co_(Y)M4_(α)  (4)

where R is at least one element selected from rare earth elements, therare earth elements including Y and a Ce content of the R being lowerthan 20% by weight including 0% by weight, M4 is at least one elementselected from the group consisting of Mn, Fe, Al, Ga, Zn, Sn, Cu, Si, B,Nb, W, Ti, Zr, In, Mo, V, Cr, P and S, the atomic ratios of a, X, Y, Zand α are respectively satisfy conditions of: 0.15≦a≦0.33, 0.06≦X≦0.15,0≦Y≦0.2, 3.15<Z≦3.55 and 0≦α<0.135.

According to a twenty-third aspect of the present invention, there isprovided a hybrid car, comprising a electric driving mechanism and apower source for driving the electrical driving mechanism:

wherein the power source comprises a secondary battery comprising apositive electrode, a negative electron containing a hydrogen absorbingalloy, and an alkaline electrolyte; and

wherein the hydrogen absorbing alloy has a composition represented bygeneral formula (4) given above, and a parallel growth regionprecipitates in at least one crystal grain of a principal phase, theparallel growth region having a crystal structure differing from acrystal structure of the principal phase.

Further, according to a twenty-fourth aspect of the present invention,there is provided an electric automobile comprising a secondary battery:

wherein the secondary battery comprises a positive electrode, a negativeelectron containing a hydrogen absorbing alloy, and an alkalineelectrolyte; and

wherein the hydrogen absorbing alloy has a composition represented bygeneral formula (4) given above, and a parallel growth regionprecipitates in at least one crystal grain of a principal phase, theparallel growth region having a crystal structure differing from acrystal structure of the principal phase.

The hydrogen absorbing alloy of the present invention, the secondarybattery comprising the negative electrode containing the particularhydrogen absorbing alloy, and the hybrid car and the electric automobileeach comprising the particular secondary battery will now be described.

Additional objects and advantages of the invention will be set forth inthe description which follows, and in part will be obvious from thedescription, or may be learned by practice of the invention. The objectsand advantages of the invention may be realized and obtained by means ofthe instrumentalities and combinations particularly pointed outhereinafter.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

The accompanying drawings, which are incorporated in and constitute apart of the specification, illustrate embodiments of the invention, andtogether with the general description given above and the detaileddescription of the embodiments given below, serve to explain theprinciples of the invention.

FIG. 1 is a schematic drawing for explaining an example of an electrondiffraction analysis pattern;

FIG. 2 is an oblique view, partly broken away, showing as an example theconstruction of a secondary battery of the present invention;

FIG. 3 is a graph showing the X-ray diffraction patterns using a CuKαray in respect of the hydrogen absorbing alloys for Examples 1, 13 and14 of the present invention;

FIG. 4 is a transmission electron micrograph showing the electrondiffraction analysis pattern of the hydrogen absorbing alloy for Example14 of the present invention;

FIG. 5 is a schematic drawing for explaining the micrograph shown inFIG. 4; and

FIG. 6 is a transmission electron micrograph of the hydrogen absorbingalloy for Example 23 of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

<First Hydrogen Absorbing Alloy>

The first hydrogen absorbing alloy of the present invention comprises asa principal phase at least one phase selected from the group consistingof a first phase having a crystal structure of a hexagonal systemexcluding a phase having a CaCu₅ type structure and a second phasehaving a rhombohedral crystal structure, wherein a content of a phasehaving a crystal structure of AB₂ type is not higher than 10% by volumeincluding 0% by volume in the hydrogen absorbing alloy and the hydrogenabsorbing alloy has a composition represented by general formula (1)given below:R_(1-a-b)Mg_(a)T_(b)Ni_(Z-X-Y-α)M1_(X)M2_(Y)Mn_(α)  (1)

where R represents at least one element selected from rare earthelements, the rare earth elements including Y, T represents at least oneelement selected from the group consisting of Ca, Ti, Zr and Hf, M1represents at least one element selected from the group consisting of Coand Fe, M2 represents at least one element selected from the groupconsisting of Al, Ga, Zn, Sn, Cu, Si, B, Nb, W, Mo, V, Cr, Ta, Li, P andS, the atomic ratios of a, b, X, Y, α and z are respectively satisfyconditions of: 0.15≦a≦0.37, 0≦b≦0.3, 0≦X≦1.3, 0≦Y≦0.5, 0≦α<0.135 and2.5≦Z≦4.2.

The crystal structure of A_(n)B_(m) type (where each of n and mrepresents a natural number) means a crystal structure of a phase havinga composition represented by A_(n)B_(m), wherein R, Mg and T included ingeneral formula (1) given above belong to the elements A, and Ni, M1, M2and Mn included in general formula (1) belong to the elements B.

It is desirable for the first phase group to include a phase having aCe₂Ni₇ structure, a phase having a CeNi₃ structure, and a phase having acrystal structure similar to the Ce₂Ni₇ structure or the CeNi₃structure. On the other hand, it is desirable for the second phase groupto include a phase having a Gd₂Co₇ structure, a phase having a PuNi₃structure, and a phase having a crystal structure similar to the Gd₂Co₇structure or the PuNi₃ structure. Incidentally, the phase having acrystal structure similar to the Ce₂Ni₇ structure, the CeNi₃ structure,the Gd₂Co₇ structure or the PuNi₃ structure, which is hereinafterreferred to as a “similar crystal phase”, means a phase satisfyingcondition (a) or (b) given below:

(a) A phase in which the main peak appearing in the X-ray diffractionpattern resembles the main peak appearing in the X-ray diffractionpattern of a normal structure. Particularly, it is desirable for thesimilar crystal phase to have a crystal structure that can be defined bythe plane index (Miller index) of the Ce₂Ni₇ structure, the CeNi₃structure, the Gd₂Co₇ structure or the PuNi₃ structure. Particularly, itis desirable for the similar crystal phase to have a crystal structuredescribed in item (1) or (2) given below:

(1) A crystal structure in which the peak having the highest intensityappears at a value of 2θ falling within a range of 42.1°±1° in an X-raydiffraction pattern using a CuKα-ray, θ representing the Bragg angle,and the intensity ratio defined by formula (I) given below is not higherthan 80%:I₃/I₄  (I)

Where I₄ represents an intensity of a peak having a highest intensity inthe X-ray diffraction pattern using CuKα-ray, I₃ represents an intensityof a peak appearing at a value of 2θ falling within a range from 31° to34° in the X-ray diffraction pattern noted above.

(2) A crystal structure in which a peak having a highest intensityappears at a value of 2θ falling within a range of 42.1°±1° in an X-raydiffraction pattern using a CuKα-ray, and a peak appearing at a value of2θ that falls within a range from 31° to 34 in the X-ray diffractionpattern is split into two or more.

(b) A phase in which a supper lattice reflection spot is present in aSn-equally divided point of the distance |G_(00L)| between the primitivelattice reflection spot (00L) and the origin (000) in the electrondiffraction analysis pattern photographed by a transmission electronmicroscope, where each of L and n represents a natural number.

It is desirable for the distance |G_(00L)| to fall within a range offrom 0.385 nm⁻¹ to 0.413 nm⁻¹, and most desirably to be 0.4 nm⁻¹.

For example, when n is 1, each of 4 points that equally divide thedistance |G_(00L)| between the primitive lattice reflection spot (00L)and the origin (000) into five sections is the supper lattice reflectionspot.

Incidentally, when it comes to a hydrogen absorbing alloy having aCe₂Ni₇ type crystal structure or a Gd₂Co₇ type crystal structure, thesupper lattice reflection spot is present in the points that equallydivide the distance |G_(00L)| between the primitive lattice reflectionspot (00L) and the origin (000) into three sections in the electrondiffraction analysis pattern. On the other hand, when it comes to ahydrogen absorbing alloy having a CeNi₃ type crystal structure or aPuNi₃ type crystal structure, the supper lattice reflection spot ispresent in the point that equally divides the distance |G_(00L)| betweenthe primitive lattice reflection spot (00L) and the origin (000) intotwo sections in the electron diffraction analysis pattern.

Among the similar crystal phases pointed out above, it is desirable touse the similar crystal phase satisfying both conditions (a) and (b)given above.

It is desirable for the principal phase of the hydrogen absorbing alloyof the present invention to be at least one phase selected from thegroup consisting of a phase having a PuNi₃ structure, a phase having acrystal structure similar to the PuNi₃ structure, a phase having aCe₂Ni₇ structure, and a phase having a crystal structure similar to theCe₂Ni₇ structure. Particularly, it is desirable for the principal phaseto be at least one phase selected from the group consisting of a phasehaving a Ce₂Ni₇ structure, a phase having a crystal structure similar tothe Ce₂Ni₇ structure.

The term “principal phase” used herein means at least one phase selectedfrom the group consisting of the first phase referred to previously andthe second phase referred to previously, which occupies the largestvolume in the hydrogen absorbing alloy, or which occupies the largestarea in a cross section of the hydrogen absorbing alloy. Particularly,it is desirable for at least one phase selected from the groupconsisting of the first phase and the second phase to occupy at least50% by volume of the hydrogen absorbing alloy. If the volume ratio ofthe particular phase is lower than 50% by volume, it is possible for thehydrogen storage capacity to be lowered. It follows that the secondarybattery comprising the negative electrode containing the particularhydrogen absorbing alloy tends to incur a decrease in the dischargecapacity or to incur shortening in the charge-discharge cycle life. Itis more desirable for the volume ratio of the principal phase to be atleast 60% by volume, more desirably at least 70% by volume.

It is acceptable for the hydrogen absorbing alloy of the presentinvention to contain a phase having an AB₅ type crystal structure suchas a CaCu₅ type structure, a phase having an AB₂ type crystal structuresuch as a MgCu₂ type structure, or a phase having both an AB₅ typecrystal structure and an AB₂ type crystal structure in addition to atleast one phase selected from the group consisting of the first phaseand the second phase.

Particularly, it is desirable for the volume ratio of the phase havingan AB₂ crystal structure to be not higher than 10% by volume including0% by volume. If the volume ratio of the particular phase exceeds 10% byvolume, the hydrogen absorption-desorption characteristics of thehydrogen absorbing alloy are lowered. It follows that the secondarybattery comprising the negative electrode containing the particularhydrogen absorbing alloy incurs a decrease in the discharge capacity. Itis more desirable for the volume ratio of the particular phase to be nothigher than 5% by volume.

Further, it is desirable for the volume ratio of the phase having an AB₅crystal structure to be not higher than 10% by volume, more desirably tobe not higher than 5% by volume.

The volume ratio of the desired phase in the hydrogen absorbing alloysuch as the principal phase, the AB₂ type phase or the AB₅ type phase ismeasured as follows. Specifically, scanning electron micrographs ofoptional five view fields are prepared. Then, an area ratio of thedesired phase relative to the alloy area within the view field arecalculated for each micrograph with the alloy area being 100%. Theaverage value of the area ratios thus obtained are calculated so as toobtain the volume ratio of the desired phase within the hydrogenabsorbing alloy. It should be noted that, if the hydrogen absorbingalloy is manufactured by a rapid solidification process, the crystalgrain size is rendered very small, i.e., about 1 μm or less. In thiscase, it is difficult to observe the desired phase by the scanningelectron micrograph. In such a case, a transmission electron microscopeis used in place of the scanning electron microscope.

In order to lower the manufacturing cost of the electrode containing thehydrogen absorbing alloy, it is desirable for R included in generalformula (1) of the hydrogen absorbing alloy to be at least one elementselected from the group consisting of La, Ce, Pr, Nd and Y.Particularly, it is desirable to use a mish metal, which is a mixture ofrare earth elements, as R. The mish metal that can be used in thepresent invention includes a Ce-rich mish metal (Mm) and a La-rich mishmetal (Lm).

It is desirable for R to include La. The La content should desirablyfall within a range of from 45% by weight to 95% by weight. If the Lacontent is lower than 45% by weight, the hydrogen absorbing alloy tendsto be pulverized by the repetition of the hydrogenabsorption-desorption. Therefore, the secondary battery comprising thenegative electrode containing the particular hydrogen absorbing alloytends to incur the shortening of the cycle life. On the other hand, ifthe La content exceeds 95% by weight, the equilibrium pressure of thehydrogen absorbing alloy tends to be lowered, with the result that thesecondary battery comprising the negative electrode containing theparticular hydrogen absorbing alloy tends to incur a decrease of thedischarge voltage. It is more desirable for the La content to exceed 60%by weight and to be not higher than 90% by weight.

Where Ce is contained in R, it is desirable for the Ce content of R tobe lower than 20% by weight. If the Ce content is not lower than 20% byweight, phases other than the desired phase, e.g., the CaCu₅ type phase,tend to be precipitated in a large amount so as to decrease the hydrogenstorage capacity. It is more desirable for the Ce content to be lowerthan 18% by weight, furthermore desirably, to be lower than 16% byweight.

In the present invention, the atomic ratio “a” in general formula (1) ofthe hydrogen absorbing alloy is defined to fall within a range of from0.15 to 0.37. If the atomic ratio “a” falls within the range specifiedin the present invention, the hydrogen storage capacity is increased anddesorption of hydrogen is facilitated, thereby improving the dischargecapacity of the secondary battery. If the atomic ratio “a” is lower than0.15, the hydrogen desorbing characteristics of the hydrogen absorbingalloy are deteriorated. On the other hand, if the atomic ratio “a”exceeds 0.37, the hydrogen storage capacity is markedly decreased,resulting in failure to obtain a secondary battery having a largedischarge capacity. It is more desirable for the atomic ratio “a” tofall within a range of not less than 0.15 and not more than 0.35, moredesirably, a range of not less than 0.15 and not more than 0.32, andmost desirably, a range of not less than 0.17 and not more than 0.3.

The hydrogen absorbing alloy of the present invention contains anelement T. Where T is contained in the hydrogen absorbing alloy, thecharacteristics such as the hydrogen desorption rate can be improved orthe pulverization of the hydrogen absorbing alloy accompanying thehydrogen absorption-desorption can be suppressed without markedlydecreasing the hydrogen storage capacity of the hydrogen absorbingalloy.

The atomic ratio “b” in general formula (1) of the hydrogen absorbingalloy is defined to fall within a range of from 0 to 0.3. If the atomicratio “b” exceeds 0.3, it is impossible to obtain the effects describedabove, i.e., the effects of improving the hydrogen desorptioncharacteristics and of suppressing the pulverization of the hydrogenabsorbing alloy, with the result that the secondary battery comprisingthe negative electrode containing the particular hydrogen absorbingalloy incurs a decrease in the discharge capacity. It is more desirablefor the atomic ratio “b” to fall within a range of not smaller than 0and not larger than 0.2, more desirably a range of not smaller than 0and not larger than 0.1.

The hydrogen absorbing alloy of the present invention contains anelement M1. Where M1 is contained in the hydrogen absorbing alloy, thehydrogen absorption-desorption characteristics such as the hydrogenabsorption-desorption rate of the hydrogen absorbing alloy can beimproved. It is considered reasonable to understand that the M1 additionfacilitates the diffusion of hydrogen in the hydrogen absorbing alloy orfacilitates the hydrogen absorption-desorption of the hydrogen absorbingalloy. It should also be noted that a secondary battery comprising thenegative electrode containing the particular hydrogen absorbing alloypermits improving the initial activity characteristics.

The atomic ratio “X” of general formula (1) of the hydrogen absorbingalloy should not exceed 1.3. If the atomic ratio “X” exceeds 1.3, thecycle life of the secondary battery is lowered. It is more desirable forthe atomic ratio “X” to fall within a range of not smaller than 0 andnot larger than 0.3.

The hydrogen absorbing alloy of the present invention contains anelement M2. Where M2 is contained in the hydrogen absorbing alloy, thehydrogen absorption-desorption characteristics such as the hydrogenabsorption-desorption rate of the hydrogen absorbing alloy can beimproved. It is considered reasonable to understand that the M2 additionfacilitates the diffusion of hydrogen in the hydrogen absorbing alloy orfacilitates the hydrogen absorption-desorption of the hydrogen absorbingalloy. It should also be noted that a secondary battery comprising thenegative electrode containing the particular hydrogen absorbing alloypermits drastically improving the cycle characteristics.

The atomic ratio “Y” of general formula (1) of the hydrogen absorbingalloy should not exceed 0.5. If the atomic ratio “Y” exceeds 0.5, thedischarge capacity of the secondary battery is lowered. It is moredesirable for the atomic ratio “Y” to fall within a range of not smallerthan 0 and not larger than 0.3, furthermore desirably a range of notsmaller than 0.01 and not larger than 0.2.

The atomic ratio “α” of general formula (1) of the hydrogen absorbingalloy should fall within a range of not smaller than 0 and less than0.135. If the atomic ratio “α” is not smaller than 0.135, the hydrogenequilibrium pressure is lowered and, at the same time, the reversibilityis deteriorated in the hydrogen absorption-desorption reaction. Also,the secondary battery comprising the negative electrode containing theparticular hydrogen absorbing alloy is rendered low in its dischargevoltage, leading to a small discharge capacity. It is more desirable forthe atomic ratio “α” to fall within a range of not smaller than 0 andnot larger than 0.13, furthermore desirably a range of not smaller than0 and not larger than 0.1.

The atomic ratio “Z” of general formula (1) of the hydrogen absorbingalloy should fall within a range of from 2.5 to 4.2. If the atomic ratio“Z” is lower than 2.5, the phase having an AB₂ type crystal structuresuch as the MgCu₂ structure constitutes the principal phase. On theother hand, if the atomic ratio “Z” exceeds 4.2, the phase having an AB₅type crystal structure such as the CaCu₅ structure constitutes theprincipal phase. It follows that the secondary battery comprising thenegative electrode containing the hydrogen absorbing alloy having theatomic ratio “Z” lower than 2.5 or exceeding 4.2 is lowered in itsdischarge capacity and shortened in its cycle life. It is more desirablefor the atomic ratio “Z” to fall within a range of not less than 2.5 andnot larger than 4, more desirably a range of not less than 3 and notlarger than 3.8, and most desirably a range of not less than 3 and notlarger than 3.7.

Particularly, it is desirable for the hydrogen absorbing alloy to haveatomic ratios of “a”, “X”, “Y” and “Z” are respectively satisfyconditions: 0.15≦a≦0.35, 0≦X≦0.3, 0≦Y≦0.3, 2.5≦Z≦4. A secondary batterycomprising the negative electrode containing the particular hydrogenabsorbing alloy makes it possible to markedly improve the dischargecapacity and the cycle life.

It is possible for the hydrogen absorbing alloy of the present inventionto contain additional elements such as C, N, O and F as impurities asfar as the amounts of these impurity elements are not so large as toimpair the characteristics of the hydrogen absorbing alloy of thepresent invention. Incidentally, it is desirable for the amount of eachof these impurity elements to be not larger than 1% by weight.

The first hydrogen absorbing alloy of the present invention can bemanufactured by, for example, a sintering method, a high frequencyinduction melting method, or a rapid solidification process. It isdesirable to apply a heat treatment to the resultant hydrogen absorbingalloy.

<Second Hydrogen Absorbing Alloy>

The second hydrogen absorbing alloy of the present invention contains asa principal phase at least one phase selected from the group consistingof a first phase having a crystal structure of a hexagonal systemexcluding a phase having a CaCu₅ type structure and a second phasehaving a rhombohedral crystal structure,

wherein a parallel growth region precipitates in at least one crystalgrain of the principal phase, the parallel growth region having acrystal structure that differs from a crystal structure of the principalphase, and the hydrogen absorbing alloy has a composition represented bygeneral formula (1) given previously.

The first phase and the second phase referred to above are similar tothose described previously in conjunction with the first hydrogenabsorbing alloy of the present invention. Also, the term “principalphase” referred to above is equal to that defined previously inconjunction with the first hydrogen absorbing alloy of the presentinvention.

It is desirable for the principal phase of the second hydrogen absorbingalloy of the present invention to be at least one phase selected fromthe group consisting of a phase having a PuNi₃ structure, a phase havinga crystal structure similar to the PuNi₃ structure, a phase having aCe₂Ni₇ structure, and a phase having a crystal structure similar to theCe₂Ni₇ structure. Particularly, it is desirable for the principal phasenoted above to be at least one phase selected from the group consistingof a phase having a Ce₂Ni₇ structure, and a phase having a crystalstructure similar to the Ce₂Ni₇ structure.

It is desirable for the volume ratio of at least one phase selected fromthe group consisting of the first phase and the second phase based onthe hydrogen absorbing alloy to be at least 50% by volume for the reasondescribed previously in conjunction with the first hydrogen absorbingalloy of the present invention. It is more desirable for the volumeratio noted above to be at least 60% by volume, more desirably at least70% by volume.

The term “parallel growth” noted above means that a plurality ofcrystals grow substantially in parallel with at least one axis, asdescribed in “McGraw-Hill DICTIONARY OF SCIENTIFIC AND TECHNICAL TERMS,Published by K.K. Nikkan Kogyyo Shinbun-sha on Jan. 30, 1980, page1280”.

The parallel growth can be observed by photographing with amagnification of 10,000 to 500,000 a transmission electron microscopicimage at (1,0,0) plane of a crystal grain of the alloy.

It is desirable for the parallel growth region excluding the principalphase to be formed of at least one kind of a region selected from thegroup consisting of a region having an AB₃ type crystal structure, aregion having an A₂B₇ crystal structure, and a region having an A₅B₁₉type crystal structure. The AB₃ type crystal structure includes, forexample, a PuNi₃ type and a CeNi₃ type. On the other hand, the A₂B₇ typecrystal structure includes, for example, a Ce₂Ni₇ type.

The volume ratio of the parallel growth region of the at least onecrystal grain can be measured as follows. Specifically, transmissionelectron micrographs of optional 30 view fields having a magnificationof 20,000 to 70,000 are prepared. An area of the parallel growth regionexcluding the principal phase is measured for each micrograph. Then, anarea ratio of the parallel growth region excluding the principal phaserelative to the alloy area within the view field, with the alloy areabeing set at 100%, was calculated in respect of each micrograph. Theaverage value of the area ratios thus obtained is calculated so as toobtain the volume ratio of the parallel growth region of the at leastone crystal grain.

It is desirable to set the volume ratio of the parallel growth region ofthe at least one crystal grain at 40% by volume or less. If the volumeratio of the parallel growth region exceeds 40% by volume, it isrendered difficult to improve the hydrogen desorption characteristicsand the cycle characteristics of the hydrogen absorbing alloy, with theresult that there is a possibility that it is rendered difficult toprovide a secondary battery having a large discharge capacity andexcellent in the charge-discharge cycle life. It is more desirable forthe volume ratio of the parallel growth region of the crystal grain tobe not higher than 35% by volume, furthermore desirably not higher than30% by volume.

A ratio of the number of crystal grains whose volume ratio of theparallel growth region is not higher than 40% by volume to the totalnumber of crystal grains can be measured as follows. Specifically, thetransmission electron microscopic images of the (1,0,0) plane of thecrystal grain in optional 30 view fields are photographed with amagnification of 20,000 to 70,000. An area of the parallel growth regionexcluding the principal phase is measured for each micrograph. Then, anarea ratio of the parallel growth region excluding the principal phaserelative to the alloy area within the view field, with the alloy areabeing set at 100%, is calculated in respect of each micrograph. A ratioof the number of view fields whose volume ratio of the parallel growthregion is not higher than 40% by volume to the number of 30 view fieldsis calculated so as to determine the ratio of the number of crystalgrains whose volume ratio of the parallel growth region is not higherthan 40% by volume to the total number of crystal grains of the alloy.

It is desirable for the number of crystal grains whose volume ratio ofthe parallel growth region is not higher than 40% by volume to be notsmaller than 60% of the total number of crystal grains of the alloy. Ifthe number of crystal grains whose volume ratio of the parallel growthregion is not higher than 40% by volume is smaller than 60% of the totalnumber of crystal grains of the alloy, it is rendered difficult toimprove the hydrogen desorption characteristics and the cyclecharacteristics of the hydrogen absorbing alloy, with the result thatthere is a possibility that it is rendered difficult to provide asecondary battery having a large discharge capacity and excellent in thecharge-discharge cycle life. It is more desirable for the number ofcrystal grains whose volume ratio of the parallel growth region is nothigher than 40% by volume to be not smaller than 65%, furthermoredesirably not smaller than 70%, of the total number of crystal grains ofthe alloy.

It is acceptable for the second hydrogen absorbing alloy of the presentinvention to contain a phase having an AB₅ type crystal structure suchas CaCu₅ structure, a phase having an AB₂ type crystal structure such asMgCu₂ structure, or both a phase having an AB₅ type crystal structureand a phase having an AB₂ type crystal structure in addition to at leastone phase selected from the group consisting of the first phase and thesecond phase.

Particularly, it is desirable for a phase having an AB₂ type crystalstructure to be present in an amount not larger than 10% by volume,including 0% by volume. If the amount of the phase having an AB₂ crystalstructure exceeds 10% by volume, the hydrogen absorption-desorptioncharacteristics of the hydrogen absorbing alloy tend to be lowered, withthe result that it is possible for the secondary battery comprising thenegative electrode containing the particular hydrogen absorbing alloy tobe rendered poor in its discharge capacity. It is more desirable for theamount of the phase having the particular crystal structure to be notlarger than 5% by volume.

Further, it is desirable for the amount of a phase having an AB₅ typecrystal structure to be not larger than 10% by volume, more desirably tobe not larger than 5% by volume.

It is acceptable for the second hydrogen absorbing alloy of the presentinvention to contain additional elements such as C, N, O and F asimpurities as far as the amounts of these additional elements are not solarge as to impair the characteristics of the hydrogen absorbing alloyof the present invention. Incidentally, it is desirable for the amountof each of these impurity elements to be not larger than 1% by weight.

The second hydrogen absorbing alloy of the present invention can bemanufactured by, for example, a sintering method, a high frequencyinduction melting method, or a rapid solidification process. It isdesirable to apply a heat treatment to the resultant hydrogen absorbingalloy.

<Third Hydrogen Absorbing Alloy>

The third hydrogen absorbing alloy of the present invention has lessthan 0.15, including 0, of an intensity ratio calculated by formula (2)given below:I₁/I₂  (2)

where I₂ is an intensity of a strongest peak in a X-ray diffractionpattern using a CuKα ray, and I₁ is an intensity of a strongest peakappearing at a value of 2θ that falls within a range of from 8° to 13°in the X-ray diffraction pattern, θ being a Bragg angle.

Incidentally, the intensity ratio (I₁/I₂) of zero (0) means that a peakwas not detected at the value of 2θ falling within a range of from 8° to13°. Also, where one peak appears at the value of 2θ falling within arange of from 8° to 13°, the intensity of this peak is set at I₁. On theother hand, where a plurality of peaks appear respectively at the valueof 2θ falling within a range of from 8° to 13° and each of the pluralityof peaks have a same intensity, the intensity of an optional peak of theplural peaks is set at I₁.

In the third hydrogen absorbing alloy of the present invention, anamount of a phase having an AB₂ type crystal structure is not largerthan 10% by volume, including 0% by volume.

Further, the third hydrogen absorbing alloy of the present invention hasa composition represented by general formula (3) given below:R_(1-a-b)Mg_(a)T_(b)Ni_(Z-X)M3_(X)  (3)

where R represents at least one element selected from rare earthelements, the rare earth elements including Y, T represents at least oneelement selected from the group consisting of Ca, Ti, Zr and Hf, M3represents at least one element selected from the group consisting ofCo, Mn, Fe, Al, Ga, Zn, Sn, Cu, Si, B, Nb, W, Mo, V, Cr, Ta, Li, P andS, the atomic ratios of a, b, X and z are respectively satisfyconditions: 0.15≦a≦0.37, 0≦b≦0.3, 0≦X≦2 and 2.5≦Z≦4.2.

The crystal structure of A_(n)B_(m) type (where each of n and mrepresents a natural number) means a crystal structure of a phase havinga composition represented by A_(n)B_(m), wherein R, Mg and T included ingeneral formula (3) given above belong to the elements A, and Ni and M3included in general formula (3) belong to the elements B.

If the intensity ratio (I₁/I₂) noted above exceeds 0.15, the hydrogenabsorbing characteristics of the hydrogen absorbing alloy are lowered,with the result that the secondary battery comprising the negativeelectrode containing the particular hydrogen absorbing alloy is renderedpoor in the discharge capacity and the cycle life. It is more desirablefor the intensity ratio (I₁/I₂) to be not higher than 0.1, furthermoredesirably not higher than 0.05.

If the amount of the phase having the AB₂ type crystal structure exceeds10% by volume in the third hydrogen absorbing alloy of the presentinvention, the hydrogen absorbing/desorbing characteristics of thehydrogen absorbing alloy are lowered, with the result that the secondarybattery comprising the negative electrode containing the particularhydrogen absorbing alloy is rendered poor in the discharge capacity. Itis more desirable for the amount of the phase having the AB₂ typecrystal structure to be not larger than 5% by volume.

It is desirable for the third hydrogen absorbing alloy of the presentinvention to contain a phase having an AB₅ type crystal structure in anamount not larger than 10% by volume, more desirably not larger than 5%by volume.

It is desirable for the third hydrogen absorbing alloy of the presentinvention to contain as a principal phase at least one kind of a phaseselected from the group consisting of a first phase having a crystalstructure of a hexagonal system, excluding a phase having a CaCu₅ typestructure, and a second phase having a rhombohedral crystal system. Theterm “principal phase” noted above is equal to that defined previouslyin conjunction with the first hydrogen absorbing alloy of the presentinvention.

The first phase and the second phase referred to above are equal tothose described previously in conjunction with the first hydrogenabsorbing alloy of the present invention.

It is desirable for the principal phase of the third hydrogen absorbingalloy of the present invention to be at least one phase selected fromthe group consisting of a phase having a PuNi₃ structure, a phase havinga crystal structure similar to the PuNi₃ structure, a phase having aCe₂Ni₇ structure, and a phase having a crystal structure similar to theCe₂Ni₇ structure. Particularly, it is desirable for the principal phasenoted above to be at least one phase selected from the group consistingof a phase having a Ce₂Ni₇ structure, and a phase having a crystalstructure similar to the Ce₂Ni₇ structure.

It is desirable for the volume ratio of at least one phase selected fromthe group consisting of the first phase and the second phase based onthe hydrogen absorbing alloy to be at least 50% by volume. If the volumeratio of the particular phase noted above is lower than 50% by volume,the hydrogen storage capacity of the hydrogen absorbing alloy tends tobe lowered, with the result that the secondary battery comprising thenegative electrode containing the particular hydrogen absorbing alloytends to be rendered poor in its discharge capacity or tends to berendered shorter in the charge-discharge cycle life. It is moredesirable for the volume ratio of the particular phase noted above to benot lower than 60% by volume, furthermore desirably not lower than 70%by volume.

In order to lower the manufacturing cost of the electrode containing thehydrogen absorbing alloy, it is desirable for R included in generalformula (3) of the hydrogen absorbing alloy to be at least one elementselected from the group consisting of La, Ce, Pr, Nd and Y.Particularly, it is desirable to use a mish metal, which is a mixture ofrare earth elements, as R. The mish metal that can be used in thepresent invention includes a Ce-rich mish metal (Mm) and a La-rich mishmetal (Lm).

It is desirable for R to include La. The La content should desirablyfall within a range of from 45% by weight to 95% by weight. If the Lacontent is lower than 45% by weight, the hydrogen absorbing alloy tendsto be pulverized by the repetition of the hydrogenabsorption-desorption. Therefore, the secondary battery comprising thenegative electrode containing the particular hydrogen absorbing alloytends to incur the shortening of the cycle life. On the other hand, ifthe La content exceeds 95% by weight, the equilibrium pressure of thehydrogen absorbing alloy tends to be lowered, with the result that thesecondary battery comprising the negative electrode containing theparticular hydrogen absorbing alloy tends to incur a decrease of thedischarge voltage. It is more desirable for the La content to exceed 60%by weight and to be not higher than 90% by weight.

Where Ce is contained in R, it is desirable for the Ce content of R tobe lower than 20% by weight. If the Ce content is not lower than 20% byweight, phases other than the desired phase, e.g., the CaCu₅ type phase,tend to be precipitated in a large amount so as to decrease the hydrogenstorage capacity. It is more desirable for the Ce content to be lowerthan 18% by weight, furthermore desirably, to be lower than 16% byweight.

In the present invention, the atomic ratio “a” in general formula (3) ofthe hydrogen absorbing alloy is defined to fall within a range of from0.15 to 0.37. If the atomic ratio “a” falls within the range specifiedin the present invention, the hydrogen storage capacity is increased anddesorption of hydrogen is facilitated, thereby improving the dischargecapacity of the secondary battery. If the atomic ratio “a” is lower than0.15, the hydrogen desorbing characteristics of the hydrogen absorbingalloy are deteriorated. On the other hand, if the atomic ratio “a”exceeds 0.37, the hydrogen storage capacity is markedly decreased,resulting in failure to obtain a secondary battery having a largedischarge capacity. It should be noted that the intensity of the peakappearing at the value of 2θ falling within a range of from 8° to 13° inthe X-ray diffraction pattern is increased with increase in the atomicratio “a”, leading to an increase in the intensity ratio (I₁/I₂). Itfollows that it is more desirable for the atomic ratio “a” to fallwithin a range of not less than 0.15 and not larger than 0.35, moredesirably, a range of not less than 0.15 and not larger than 0.32, andmost desirably, a range of not less than 0.17 and not larger than 0.3.

The hydrogen absorbing alloy of the present invention contains anelement T. Where T is contained in the hydrogen absorbing alloy, thecharacteristics such as the hydrogen desorption rate can be improved orthe pulverization of the hydrogen absorbing alloy accompanying thehydrogen absorption-desorption can be suppressed without markedlydecreasing the hydrogen storage capacity of the hydrogen absorbingalloy.

The atomic ratio “b” in general formula (3) of the hydrogen absorbingalloy is defined to fall within a range of from 0 to 0.3. If the atomicratio “b” exceeds 0.3, it is impossible to obtain the effects describedabove, i.e., the effects of improving the hydrogen desorptioncharacteristics and of suppressing the pulverization of the hydrogenabsorbing alloy, with the result that the secondary battery comprisingthe negative electrode containing the particular hydrogen absorbingalloy incurs a decrease in the discharge capacity. It is more desirablefor the atomic ratio “b” to fall within a range of not less than 0 andnot larger than 0.2, more desirably a range of not less than 0 and notlarger than 0.1.

The third hydrogen absorbing alloy of the present invention contains anelement M3. Where M3 is contained in the hydrogen absorbing alloy, thehydrogen absorption-desorption characteristics such as the hydrogenabsorption-desorption rate of the hydrogen absorbing alloy can beimproved. It is considered reasonable to understand that the M3 additionfacilitates the diffusion of hydrogen in the hydrogen absorbing alloy orfacilitates the hydrogen absorption-desorption of the hydrogen absorbingalloy. It should also be noted that a secondary battery comprising thenegative electrode containing the particular hydrogen absorbing alloypermits improving the charge-discharge cycle characteristics.

The atomic ratio “X” of general formula (3) of the hydrogen absorbingalloy should not exceed 2.0. If the atomic ratio “X” exceeds 2.0, thedischarge capacity of the secondary battery is lowered. It is moredesirable for the atomic ratio “X” to fall within a range of from 0 to0.5.

The atomic ratio “Z” of general formula (3) of the hydrogen absorbingalloy should fall within a range of from 2.5 to 4.2. If the atomic ratio“Z” is lower than 2.5, a large amount of hydrogen is irreversible in thehydrogen absorbing alloy, with the result that the hydrogen desorbingrate is lowered. On the other hand, if the atomic ratio “Z” exceeds 4.2,the phase having an AB₅ type crystal structure is generated in a largeamount. It follows that the secondary battery comprising the negativeelectrode containing the particular hydrogen absorbing alloy is loweredin its discharge capacity. It is more desirable for the atomic ratio “Z”to fall within a range of from 3.0 to 4.0.

Particularly, it is desirable for the hydrogen absorbing alloy to haveatomic ratios of “a” and “X” are respectively satisfy conditions of:0.15≦a≦0.35 and 0≦X≦0.5. A secondary battery comprising the negativeelectrode containing the particular hydrogen absorbing alloy makes itpossible to markedly improve the discharge capacity and the cycle life.

It is possible for the hydrogen absorbing alloy of the present inventionto contain additional elements such as C, N, O and F as impurities asfar as the amounts of these impurity elements are not so large as toimpair the characteristics of the hydrogen absorbing alloy of thepresent invention. Incidentally, it is desirable for the amount of eachof these impurity elements to be not larger than 1% by weight.

The third hydrogen absorbing alloy of the present invention can bemanufactured by, for example, a sintering method, a high frequencyinduction melting method, or a rapid solidification process. It isdesirable to apply a heat treatment to the resultant hydrogen absorbingalloy.

<Fourth Hydrogen Absorbing Alloy>

In the fourth hydrogen absorbing alloy of the present invention, theintensity ratio calculated by formula (2) referred to previously issmaller than 0.15, including 0. Also, the fourth hydrogen absorbingalloy of the present invention has a composition represented by generalformula (3) referred to previously. Further, a parallel growth regionprecipitates in at least one crystal grain of a principal phase of thehydrogen absorbing alloy. The parallel growth region has a crystalstructure differing from a crystal structure of the principal phase.

The term “principal phase” used herein means the phase that occupies thelargest volume in the hydrogen absorbing alloy, or occupies the largestarea in a cross section of the hydrogen absorbing alloy. Particularly,it is desirable for the principal phase to occupy at least 50% by volumeof the hydrogen absorbing alloy. It is more desirable for the volumeratio of the principal phase to be at least 60% by volume, moredesirably at least 70% by volume.

The term “parallel growth” noted above means that a plurality ofcrystals grow substantially in parallel with at least one axis, asdescribed in the “McGraw-Hill DICTIONARY OF SCIENTIFIC AND TECHNICALTERMS, Published by K.K. Nikkan Kogyyo Shinbun-sha on Jan. 30, 1980,page 1280”.

The parallel growth can be observed by photographing with amagnification of 10,000 to 500,000 a transmission electron microscopicimage at (1,0,0) plane of a crystal grain of the alloy.

The parallel growth region excluding the principal phase includes thosedescribed previously in conjunction with the second hydrogen absorbingalloy of the present invention.

It is desirable to set the volume ratio of the parallel growth regionthat differs from the principal phase at 40% by volume or less of the atleast one crystal grain. If the volume ratio of the parallel growthregion exceeds 40% by volume, it is rendered difficult to improve thehydrogen desorption characteristics and the cycle characteristics of thehydrogen absorbing alloy, with the result that there is a possibilitythat it is rendered difficult to provide a secondary battery having alarge discharge capacity and excellent in the charge-discharge cyclelife. It is more desirable for the volume ratio of the parallel growthregion of the crystal grain to be not higher than 35% by volume,furthermore desirably not higher than 30% by volume.

It is desirable for the number of crystal grains whose volume ratio ofthe parallel growth region that differs from the principal phase is nothigher than 40% to be not smaller than 60% of the total number ofcrystal grains of the alloy. If the number of crystal grains is smallerthan 60% of the total number of crystal grains of the alloy, it isrendered difficult to improve the hydrogen desorption characteristicsand the cycle characteristics of the hydrogen absorbing alloy, with theresult that there is a possibility that it is rendered difficult toprovide a secondary battery having a large discharge capacity andexcellent in the charge-discharge cycle life. It is more desirable forthe number of crystal grains to be not smaller than 65%, furthermoredesirably not smaller than 70%, of the total number of crystal grains ofthe alloy.

It is desirable for the principal phase of the fourth hydrogen absorbingalloy of the present invention to be at least one kind of a phaseselected from the group consisting of a first phase in which the crystalstructure is of a hexagonal system, excluding the phase having a CaCu₅type structure, and a second phase having a rhombohedral crystalstructure.

The first phase and the second phase referred to above are equal tothose described previously in conjunction with the first hydrogenabsorbing alloy of the present invention.

It is desirable for the principal phase of the fourth hydrogen absorbingalloy of the present invention to be at least one kind of a phaseselected from the group consisting of a phase having a PuNi₃ structure,a phase having a crystal structure similar to the PuNi₃ structure, aphase having a Ce₂Ni₇ structure, and a phase having a crystal structuresimilar to the Ce₂Ni₇ structure. Particularly, it is desirable for theprincipal phase of the fourth hydrogen absorbing alloy of the presentinvention to be at least one kind of a phase selected from the groupconsisting of a phase having a Ce₂Ni₇ structure, and a phase having acrystal structure similar to the Ce₂Ni₇ structure.

It is desirable for the volume ratio of at least one phase selected fromthe group consisting of the first phase and the second phase to be notlower than 50% by volume based on the hydrogen absorbing alloy. If thevolume ratio noted above is lower than 50% by volume, the hydrogenstorage capacity of the hydrogen absorbing alloy tends to be lowered,with the result that the secondary battery comprising the negativeelectrode containing the particular hydrogen absorbing alloy tends to belowered in the discharge capacity or to be shortened in thecharge-discharge cycle life. It is more desirable for the volume ratioof the particular phase to be not lower than 60% by volume, moredesirably not lower than 70% by volume.

In the fourth hydrogen absorbing alloy of the present invention, it isdesirable for the amount of the phase having a crystal structure of AB₂type to be not larger than 10% by volume, including 0% by volume. If theamount of the particular phase exceeds 10% by volume, the hydrogenabsorption-desorption characteristics of the hydrogen absorbing alloytend to be lowered, with the result that the secondary batterycomprising the negative electrode containing the particular hydrogenabsorbing alloy tends to be lowered in its discharge capacity. It ismore desirable for the amount of the particular phase to be not largerthan 5% by volume.

In the fourth hydrogen absorbing alloy of the present invention, it isdesirable for the amount of the phase having a crystal structure of AB₅type to be not larger than 10% by volume, more desirably to be notlarger than 5% by volume.

It is acceptable for the fourth hydrogen absorbing alloy of the presentinvention to contain additional elements such as C, N, O and F asimpurities as far as the amounts of these additional elements are not solarge as to impair the characteristics of the hydrogen absorbing alloyof the present invention. Incidentally, it is desirable for the amountof each of these impurity elements to be not larger than 1% by weight.

The fourth hydrogen absorbing alloy of the present invention can bemanufactured by, for example, a sintering method, a high frequencyinduction melting method, or a rapid solidification process. It isdesirable to apply a heat treatment to the resultant hydrogen absorbingalloy.

<Fifth Hydrogen Absorbing Alloy>

The fifth hydrogen absorbing alloy of the present invention has acomposition represented by general formula (4) given below and, in thefifth hydrogen absorbing alloy of the present invention, it is desirablefor a phase having an AB₂ type crystal structure to be contained in thefifth hydrogen absorbing alloy of the present invention in an amount notlarger than 10% by volume, including 0% by volume:R_(1-a)Mg_(a)Ni_(Z-X-Y)Al_(X)Co_(Y)M4_(α)  (4)

where R represents at least one element selected from rare earthelements, the rare earth elements including Y and a Ce content of the Rbeing lower than 20% by weight including 0% by weight, M4 represents atleast one element selected from the group consisting of Mn, Fe, Al, Ga,Zn, Sn, Cu, Si, B, Nb, W, Ti, Zr, In, Mo, V, Cr, P and S, the atomicratio s of a, X, Y, Z and α are respectively satisfy conditions:0.15≦a≦0.33, 0.06≦X≦0.15, 0≦Y≦0.2, 3.15<Z≦3.55 and 0≦α<0.135.

The crystal structure of A_(n)B_(m) type (where each of n and mrepresents a natural number) means a crystal structure of a phase havinga composition represented by A_(n)B_(m), wherein R and Mg included ingeneral formula (4) given above belong to the elements A, and Ni, Al, Coand M4 included in general formula (4) belong to the elements B.

In the present invention, the atomic ratio “a” in general formula (4) ofthe hydrogen absorbing alloy is defined to fall within a range of from0.15 to 0.33. If the atomic ratio “a” is lower than 0.15, the hydrogenabsorbed in the hydrogen absorbing alloy is rendered stable, with theresult that the hydrogen is rendered unlikely to be desorbed from thehydrogen absorbing alloy. On the other hand, if the atomic ratio “a”exceeds 0.33, the phases other than the desired phase, e.g., the phaseof CaCu₅ type, tend to be precipitated easily, with the result that thehydrogen storage capacity is decreased. It is more desirable for theatomic ratio “a” to satisfy condition of: 0.17≦a≦0.31, more desirably,0.18≦a≦0.3.

In order to lower the manufacturing cost of the electrode containing thehydrogen absorbing alloy, it is desirable for R included in generalformula (4) of the hydrogen absorbing alloy to be at least one elementselected from the group consisting of La, Ce, Pr, Nd and Y.Particularly, it is desirable to use a mish metal, which is a mixture ofrare earth elements, as R.

In the present invention, the Ce amount contained in R is defined to beless than 20% by weight. If the Ce amount is 20% by weight or more,phases other than the desired phase, e.g., the phase of CaCu₅ type, isprecipitated in a large amount so as to decrease the hydrogen storagecapacity of the hydrogen absorbing alloy. It is more desirable for theCe amount to be smaller than 18% by weight, furthermore desirablysmaller than 16% by weight.

It is desirable for R to include La. The La content should desirablyfall within a range of from 45% by weight to 95% by weight. If the Lacontent is lower than 45% by weight, the hydrogen absorbing alloy tendsto be pulverized by the repetition of the hydrogenabsorption-desorption. Therefore, the secondary battery comprising thenegative electrode containing the particular hydrogen absorbing alloytends to incur the shortening of the cycle life. On the other hand, ifthe La content exceeds 95% by weight, the equilibrium pressure of thehydrogen absorbing alloy tends to be lowered, with the result that thesecondary battery comprising the negative electrode containing theparticular hydrogen absorbing alloy tends to incur a decrease of thedischarge voltage. It is more desirable for the La content to exceed 60%by weight and to be not higher than 90% by weight.

The atomic ratio “X” of general formula (4) of the hydrogen absorbingalloy should fall within a range of from 0.06 to 0.15. If the atomicratio “X” is lower than 0.06, the deterioration of the characteristicscaused by the oxidation of the hydrogen absorbing alloy under a hightemperature environment is prominently accelerated. On the other hand,if the atomic ratio “X” exceeds 1.5, it is possible for the phase otherthan the desired phase, e.g., the phase of CaCu₅ type, to beprecipitated in a large amount. It is more desirable for the atomicratio “X” to satisfy condition of: 0.07≦X≦0.13, furthermore desirably0.08≦X≦0.12.

The atomic ratio “Y” of general formula (4) of the hydrogen absorbingalloy should not exceed 0.2. Even if the atomic ratio “Y” of Co is sethigher than 0.2 in the hydrogen absorbing alloy in which the atomicratio “X” of Al falls within the range referred to above, it isimpossible to improve the corrosion resistance of the hydrogen absorbingalloy. In addition, it is disadvantageous in terms of the manufacturingcost of the hydrogen absorbing alloy to increase the atomic ratio “Y” toexceed 0.2. It is more desirable for the atomic ratio “Y” to satisfycondition of: 0≦Y≦0.18, furthermore desirably 0≦Y≦0.15.

The atomic ratio “α” of general formula (4) of the hydrogen absorbingalloy should fall within a range of not less than 0 and less than 0.135.If the atomic ratio “α” is 0.135 or more, the phase other than thedesired phase, e.g., the phase of CaCu₅ type, tends to be precipitated,with the result that it is possible for the hydrogen storage capacity ofthe hydrogen absorbing alloy to be lowered. It is more desirable for theatomic ratio “α” to satisfy condition of: 0≦α≦0.13, furthermoredesirably between 0≦α≦0.12, and most desirably between 0≦α≦0.1.

The atomic ratio “Z” of general formula (4) of the hydrogen absorbingalloy should fall within a range of higher than 3.15 and not higher than3.55. If the atomic ratio “Z” is not larger than 3.15, the Laves phasehaving an AB₂ type crystal structure tends to be precipitated, with theresult that the irreversible hydrogen is increased with progress in therepetition of the hydrogen absorption-desorption so as to markedlydecrease the hydrogen storage capacity. On the other hand, if the atomicratio “Z” exceeds 3.55, the phase other than the desired phase, e.g.,the phase of the CaCu₅ type, tends to be precipitated easily so as todecrease the hydrogen storage capacity of the hydrogen absorbing alloy.It is more desirable for the atomic ratio “Z” to satisfy condition of:3.17≦Z≦3.53, more desirably 3.18≦Z≦3.52.

If the amount of the phase having the AB₂ type crystal structure, whichis contained in the fifth hydrogen absorbing alloy of the presentinvention, exceeds 10% by volume, the hydrogen absorption-desorptioncharacteristics of the hydrogen absorbing alloy are lowered, with theresult that the secondary battery comprising the negative electrodecontaining the particular hydrogen absorbing alloy is lowered in itsdischarge capacity. It is more desirable for the amount of the phasehaving the AB₂ type crystal structure to be not larger than 5% byvolume.

It is desirable for the amount of the phase having the AB₅ type crystalstructure, which is contained in the fifth hydrogen absorbing alloy ofthe present invention, not to exceed 10% by volume, more desirably notto exceed 5% by volume.

It is desirable for the fifth hydrogen absorbing alloy of the presentinvention to contain as a principal phase at least one kind of a phaseselected from the group consisting of a first phase having a crystalstructure of the hexagonal system, excluding the phase having the CaCu₅type structure, and a second phase having a rhombohedral crystal system.The term “principal phase” noted above is equal to that definedpreviously in conjunction with the first hydrogen absorbing alloy of thepresent invention.

The first phase and the second phase referred to above are equal tothose described previously in conjunction with the first hydrogenabsorbing alloy of the present invention.

It is desirable for the principal phase of the fifth hydrogen absorbingalloy of the present invention to be at least one phase selected fromthe group consisting of a phase having a PuNi₃ structure, a phase havinga crystal structure similar to the PuNi₃ structure, a phase having aCe₂Ni₇ structure, and a phase having a crystal structure similar to theCe₂Ni₇ structure. Particularly, it is desirable for the principal phasenoted above to be at least one phase selected from the group consistingof a phase having a Ce₂Ni₇ structure, and a phase having a crystalstructure similar to the Ce₂Ni₇ structure.

It is desirable for the volume ratio of at least one phase selected fromthe group consisting of the first phase and the second phase based onthe hydrogen absorbing alloy to be at least 50% by volume. If the volumeratio of the particular phase noted above is lower than 50% by volume,the hydrogen storage capacity of the hydrogen absorbing alloy tends tobe lowered, with the result that the secondary battery comprising thenegative electrode containing the particular hydrogen absorbing alloytends to be rendered poor in its discharge capacity or tends to berendered shorter in the charge-discharge cycle life. It is moredesirable for the volume ratio of the particular phase noted above to benot lower than 60% by volume, furthermore desirably not lower than 70%by volume.

It is possible for the hydrogen absorbing alloy of the present inventionto contain additional elements such as C, N, O and F as impurities asfar as the amounts of these impurity elements are not so large as toimpair the characteristics of the hydrogen absorbing alloy of thepresent invention. Incidentally, it is desirable for the amount of eachof these impurity elements to be not larger than 1% by weight.

The fifth hydrogen absorbing alloy of the present invention can bemanufactured by, for example, a sintering method, a high frequencyinduction melting method, or a rapid solidification process. It isdesirable to apply a heat treatment to the resultant hydrogen absorbingalloy.

<Sixth Hydrogen Absorbing Alloy>

The sixth hydrogen absorbing alloy of the present invention has acomposition represented by general formula (4) referred to previously.Also, a parallel growth region precipitates in at least one crystalgrain of a principal phase of the hydrogen absorbing alloy. The parallelgrowth region has a crystal structure differing from a crystal structureof the principal phase.

The term “principal phase” denotes the phase occupying the largestvolume in the hydrogen absorbing alloy or the phase occupying thelargest area in a cross section of the hydrogen absorbing alloy.Particularly, it is desirable for the volume ratio of the principalphase in the hydrogen absorbing alloy to be not lower than 50% byvolume, more desirably not lower than 60% by volume, and furthermoredesirably not lower than 70% by volume.

The term “parallel growth” noted above means that a plurality ofcrystals grow substantially in parallel with at least one axis, asdescribed in the “McGraw-Hill DICTIONARY OF SCIENTIFIC AND TECHNICALTERMS, Published by K.K. Nikkan Kogyyo Shinbun-sha on Jan. 30, 1980,page 1280”.

The parallel growth can be observed by photographing with amagnification of 10,000 to 500,000 a transmission electron microscopicimage at (1,0,0) plane of a crystal grain of the alloy.

The parallel growth region excluding the principal phase includes thosedescribed previously in conjunction with the second hydrogen absorbingalloy of the present invention.

It is desirable for the volume ratio of the parallel growth region,which differs from the principal phase, to be not higher than 40% byvolume of the at least one crystal grain. If the volume ratio notedabove exceeds 40% by volume, it is difficult to improve the hydrogendesorption characteristics and the cycle characteristics of the hydrogenabsorbing alloy, with the result that it is difficult to provide asecondary battery having a large discharge capacity and excellent in thecharge-discharge cycle life. It is more desirable for the volume rationoted above to be not higher than 35% by volume, furthermore desirablynot higher than 30% by volume.

It is desirable for the number of crystal grains, in which the volumeratio of the parallel growth region that differs from the principalphase, is not higher than 40% by volume, to be not smaller than 60% ofthe number of all the crystal grains of the alloy. If the number ofcrystal grains noted above is smaller than 60% of the number of all thecrystal grains of the alloy, it is difficult to improve the hydrogendesorption characteristics and the cycle characteristics of the hydrogenabsorbing alloy, with the result that it is difficult to provide asecondary battery having a large discharge capacity and excellent in thecharge-discharge cycle life. It is more desirable for the number ofcrystal grains noted above to be not smaller than 65%, furthermoredesirably not smaller than 70%, of the number of all the crystal grainsof the hydrogen absorbing alloy.

It is desirable for the principal phase to be formed of at least onephase selected from the group consisting of a first phase having acrystal structure of the hexagonal system, excluding the phase havingthe CaCu₅ structure, and a second phase having a rhombohedral crystalstructure. The first phase and the second phase include those describedpreviously in conjunction with the first hydrogen absorbing alloy.

It is desirable for the principal phase of the sixth hydrogen absorbingalloy of the present invention to be at least one phase selected fromthe group consisting of a phase having a PuNi₃ structure, a phase havinga crystal structure similar to the PuNi₃ structure, a phase having aCe₂Ni₇ structure, and a phase having a crystal structure similar to theCe₂Ni₇ structure. Particularly, it is desirable for the principal phasenoted above to be at least one phase selected from the group consistingof a phase having a Ce₂Ni₇ structure, and a phase having a crystalstructure similar to the Ce₂Ni₇ structure.

It is desirable for the volume ratio of at least one phase selected fromthe group consisting of the first phase and the second phase to be notlower than 50% by volume based on the hydrogen absorbing alloy. If thevolume ratio noted above is lower than 50% by volume, the hydrogenstorage capacity of the hydrogen absorbing alloy tends to be decreased.It follows that the secondary battery comprising the negative electrodecontaining the particular hydrogen absorbing alloy tends to be low inthe discharge capacity or tends to be short in the charge-dischargecycle life. It is more desirable for the volume ratio in question to benot lower than 60% by volume, furthermore desirably not lower than 70%by volume.

In the sixth hydrogen absorbing alloy of the present invention, it isdesirable for a phase having an AB₂ type crystal structure to be presentin an amount not larger than 10% by volume, including 0% by volume. Ifthe amount of the phase having an AB₂ crystal structure exceeds 10% byvolume, the hydrogen absorption-desorption characteristics of thehydrogen absorbing alloy tend to be lowered, with the result that it ispossible for the secondary battery comprising the negative electrodecontaining the particular hydrogen absorbing alloy to be rendered poorin its discharge capacity. It is more desirable for the amount of thephase having the particular crystal structure to be not larger than 5%by volume.

Further, in the sixth hydrogen absorbing alloy of the present invention,it is desirable for the amount of a phase having an AB₅ type crystalstructure to be not larger than 10% by volume, more desirably to be notlarger than 5% by volume.

It is acceptable for the sixth hydrogen absorbing alloy of the presentinvention to contain additional elements such as C, N, O and F asimpurities as far as the amounts of these additional elements are not solarge as to impair the characteristics of the hydrogen absorbing alloyof the present invention. Incidentally, it is desirable for the amountof each of these impurity elements to be not larger than 1% by weight.

The sixth hydrogen absorbing alloy of the present invention can bemanufactured by, for example, a sintering method, a high frequencyinduction melting method, or a rapid solidification process. It isdesirable to apply a heat treatment to the resultant hydrogen absorbingalloy.

The secondary battery of the present invention will now be described.

The secondary battery of the present invention comprises an electrodegroup including a positive electrode, a negative electrode containingthe hydrogen absorbing alloy, and a separator interposed between thepositive electrode and the negative electrode, and an alkalineelectrolyte impregnated in the electrode group. The hydrogen absorbingalloy selected from the group consisting of the first to sixth hydrogenabsorbing alloys of the present invention described above can be usedfor forming the negative electrode of the secondary battery.

The positive electrode, the negative electrode, the separator and theelectrolyte included in the secondary battery of the present inventionwill now be described in detail.

1) Positive Electrode

For preparing the positive electrode, a conductive material is added to,for example, a nickel hydroxide powder used as an active material, andthe resultant mixture is kneaded together with a binder polymer andwater so as to obtain a paste. Then, a conductive substrate is filledwith the resultant paste, followed by drying the paste and pressing theconductive substrate so as to obtain the desired positive electrode.

It is possible for the nickel hydroxide powder to contain at least onecompound selected from the group consisting of zinc oxide, cobalt oxide,zinc hydroxide and cobalt hydroxide.

The conductive material used in the present invention includes, forexample, cobalt oxide, cobalt hydroxide, metallic cobalt, metallicnickel and carbon.

The binder polymer used in the present invention includes, for example,carboxymethyl cellulose, methyl cellulose, sodium polyacrylate, andpolytetrafluoro ethylene.

Further, the conductive substrate used in the present inventionincludes, for example, a mesh-shaped, a sponge-shaped, a fibrous or afelt-shaped porous metal body formed of nickel, a stainless steel or ametal plated with nickel.

2) Negative Electrode

For preparing the negative electrode, a conductive material is added toa powder of the hydrogen absorbing alloy described previously, and theresultant mixture is kneaded together with a binder polymer and water soas to obtain a paste. Then, the paste thus prepared is loaded in aconductive substrate, followed by drying the paste and pressing theconductive substrate so as to obtain a desired negative electrode.

The binder polymer similar to that described previously in conjunctionwith the positive electrode can also be used for preparing the negativeelectrode.

The conductive material used for preparing the negative electrodeincludes, for example, carbon black.

It is possible to add oxides such as Y₂O₃, Er₂O₃, Yb₂O₃, Sm₂O₃, Mn₃O₄,LiMn₂O₄, Nb₂O₅, and SnO₂ to the paste noted above. Where the negativeelectrode contains the oxides noted above, it is possible to improve thecycle life of the secondary battery under high temperatures. It ispossible to add the oxides singly or in the form of a mixture of aplurality of oxides. It is desirable for the addition amount of theoxide to fall within a range of from 0.2 to 5% by weight, more desirablyfrom 0.4 to 2% by weight, based on the weight of the hydrogen absorbingalloy.

The conductive substrate used for preparing the negative electrodeincludes, for example, a two dimensional substrate such as a punchedmetal, an expanded metal or a nickel net, and a three dimensionalsubstrate such as a felt-shaped metal porous body or a sponge-shapedmetal substrate.

3) Separator

The separator includes, for example, a polymer unwoven fabric such as apolypropylene unwoven fabric, a Nylon unwoven fabric and an unwovenfabric containing a polypropylene fiber and a Nylon fiber. Particularly,a polypropylene unwoven fabric having the surface subjected to ahydrophilic treatment is adapted for use as the separator.

4) Alkaline Electrolyte

The alkaline electrolyte used in the present invention includes, forexample, an aqueous solution of sodium hydroxide (NaOH), an aqueoussolution of lithium hydroxide (LiOH), an aqueous solution of potassiumhydroxide (KOH), a mixed solution of NaOH and LiOH, a mixed solution ofKOH and LiOH, and a mixed solution of KOH, LiOH and NaOH.

FIG. 2 shows a cylindrical alkali secondary battery as an example of thesecondary battery of the present invention.

As shown in FIG. 2, an electrode group 5 prepared by spirally winding alaminate structure consisting of a positive electrode 2, a separator 3and a negative electrode 4 is housed in a cylindrical case 1. Thenegative electrode 4 is arranged to constitute the outermostcircumferential layer of the electrode group 5 so as to be electricallyconnected to the case 1. An alkaline electrolyte is housed in the case1. A circular sealing plate 7 having a hole 6 formed in the center isarranged in the upper open portion of the case 1. An annular insulatinggasket 8 is arranged between the circumferential edge of the sealingplate 7 and the inner surface of the upper open portion of the case 1.The sealing plate 7 is hermetically fixed to the case 1 via the gasket 8by the caulking treatment to deform inward the upper open portion of thecase 1 in a manner to decrease the diameter of the upper portion of thecase 1. A positive lead 9 is connected at one end to the positiveelectrode 2 and at the other end to the lower surface of the sealingplate 7. A hat-shaped positive electrode terminal 10 is mounted to thesealing plate 7 in a manner to cover the hole 6. A safety vent 11 madeof rubber is arranged in a free space surrounded by the sealing plate 7and the positive electrode terminal 10 in a manner to close the hole 6.A circular holding plate 12 having a hole formed in the center and madeof an insulating material is arranged such that the projecting portionof the positive electrode terminal 10 projects onto the positiveelectrode terminal 10 through the hole of the holding plate 10. Further,an outer tube 13 is arranged to cover the outer periphery of the holdingplate 12, the side surface of the case 1 and the outer periphery in thebottom-portion of the case 1.

It is possible for the secondary battery of the present invention to bea rectangular alkali secondary battery constructed such that anelectrode group prepared by alternately laminating a positive electrodeand a negative electrode with a separator being interposed therebetweenis housed in a rectangular case together with an alkaline electrolyte aswell as a cylindrical alkali secondary battery shown in FIG. 2.

A hybrid car and an electric automobile of the present invention willnow be described.

The hybrid car of the present invention comprises an external combustionengine or an internal combustion engine, an electric driving mechanismprovided by, for example, a motor, and a power source for the electricaldriving mechanism. The power source comprises a secondary batterycomprising a positive electrode, a negative electrode containing ahydrogen absorbing alloy, and an alkaline electrolyte. It is possible touse at least one kind of the hydrogen absorbing alloy selected from thegroup consisting of the first to sixth hydrogen absorbing alloys of thepresent invention described previously for forming the negativeelectrode included in the secondary battery.

The hybrid car of the present invention includes a type that a dynamo isdriven by an external combustion engine or an internal combustion engineand the power generated from the dynamo and the power generated from thesecondary battery are supplied to the electric driving mechanism so asto drive the wheels of the car, and another type that the driving forcesgenerated from both the external combustion engine or the internalcombustion engine and the secondary battery are supplied appropriatelyto the electric driving mechanism so as to drive the wheels of thehybrid car.

The electric automobile of the present invention comprises a secondarybattery as a driving power source. The secondary battery comprises apositive electrode, a negative electrode containing a hydrogen absorbingalloy, and an alkaline electrolyte. It is possible to use at least onekind of the hydrogen absorbing alloy selected from the group consistingof the first to sixth hydrogen absorbing alloys of the present inventiondescribed previously for forming the negative electrode included in thesecondary battery.

The first hydrogen absorbing alloy of the present invention describedabove contains as a principal phase at least one kind of a phaseselected from the group consisting of a first phase having a crystalstructure of a hexagonal system, excluding a phase of a CaCu₅ typestructure, and a second phase having a rhombohedral crystal structure.An amount of a phase having an AB₂ type crystal structure, which iscontained in the first hydrogen absorbing alloy of the presentinvention, is not larger than 10% by volume, including 0% by volume.Also, the first hydrogen absorbing alloy of the present invention has acomposition represented by general formula (1) given previously. Thefirst hydrogen absorbing alloy of the particular construction permitsimproving the hydrogen desorption characteristics of the hydrogenabsorbing alloy, with the result that it is possible to improve thereversibility of the hydrogen absorption-desorption reaction so as toincrease the hydrogen absorption-desorption amount of the hydrogenabsorbing alloy. Also, the secondary battery comprising the negativeelectrode containing the first hydrogen absorbing alloy of the presentinvention exhibits an improved discharge capacity and an improvedcharge-discharge cycle life. It follows that the hybrid car and theelectric automobile having the particular secondary battery mountedthereon exhibit an improved running performance such as a fuel cost.

The second hydrogen absorbing alloy of the present invention comprisesas a principal phase at least one phase selected from the groupconsisting of the first phase noted above and the second phase notedabove. Also, a parallel growth region precipitates in at least onecrystal grain of the principal phase. The parallel growth region has acrystal structure differing from a crystal structure of the principalphase. Further, the second hydrogen absorbing alloy of the presentinvention has a composition represented by general formula (1) referredto above. The second hydrogen absorbing alloy of the particularconstruction makes it possible to lessen the fluctuation in the Mgcontent so as to suppress an extreme uneven distribution of Mg, with theresult that it is possible to improve the reversibility of the hydrogenabsorption-desorption reaction. It follows that the secondary batterycomprising the negative electrode containing the particular hydrogenabsorbing alloy is allowed to exhibit an improved discharge capacity anda prolonged charge-discharge cycle life. Also, the electric automobileor the hybrid car having the particular secondary battery is allowed toexhibit an improved running performance such as a fuel cost.

It should be noted that the Mg content of the parallel growth regionexcluding the principal phase is higher or lower than the Mg content ofthe principal phase depending on the situation. However, if the numberof crystal grains, in which the volume ratio of the parallel growthregion is not higher than 40% by volume, is set at a level not smallerthan 60% of the number of all the crystal grains of the alloy, it ispossible to control appropriately the fluctuation of the Mg content soas to suppress the uneven distribution of Mg, thereby further improvingthe reversibility of the hydrogen absorption-desorption reaction. Itfollows that it is possible to further improve the discharge capacityand the charge-discharge cycle life of the secondary battery comprisingthe negative electrode containing the particular hydrogen absorbingalloy. Also, it is possible to further improve the running performancesuch as the fuel cost of the hybrid car or the electric automobilehaving the particular secondary battery mounted thereto.

In the third hydrogen absorbing alloy of the present invention, theintensity ratio calculated by formula (2) referred to previously is lessthan 0.15 including 0. Also, the third hydrogen absorbing alloy of thepresent invention has a composition represented by general formula (3)given previously. Further, an amount of a phase having a AB₂ typecrystal structure, which is contained in the third hydrogen absorbingalloy of the present invention, is not larger than 10% by volumeincluding 0% by volume. The third hydrogen absorbing alloy of theparticular construction makes it possible to improve the hydrogendesorption characteristics of the hydrogen absorbing alloy, with theresult that it is possible to improve the reversibility of the hydrogenabsorption-desorption reaction so as to increase the amounts of thehydrogen absorption and the hydrogen desorption. Also, the secondarybattery comprising the negative electrode containing the particularthird hydrogen absorbing alloy of the present invention is allowed toexhibit an improved discharge capacity and an improved charge-dischargecycle life. It follows that the hybrid car or the electric automobilehaving the particular secondary battery mounted thereto is allowed toexhibit an improved running performance such as a fuel cost.

In the fourth hydrogen absorbing alloy of the present invention, theintensity ratio calculated by formula (2) referred to previously is lessthan 0.15 including 0. Also, the fourth hydrogen absorbing alloy of thepresent invention has a composition represented by general formula (3)given previously. Further, a parallel growth region precipitates in atleast one crystal grain of a principal phase of the fourth hydrogenabsorbing alloy of the present invention. The parallel growth region hasa crystal structure differing from a crystal structure of the principalphase. The fourth hydrogen absorbing alloy of the particularconstruction makes it possible to lessen the fluctuation in the Mgcontent so as to suppress an extremely uneven distribution of Mg and,thus, to improve the reversibility of the hydrogen absorption-desorptionreaction. It follows that the secondary battery comprising the negativeelectrode containing the particular hydrogen absorbing alloy is allowedto exhibit an improved discharge capacity and an improvedcharge-discharge cycle life. Also, the hybrid car or the electricautomobile having the particular secondary battery mounted thereto isallowed to exhibit an improved running performance such as a fuel cost.

It should be noted in particular that, if the number of crystal grains,in which the volume ratio of the parallel growth region is not higherthan 40% by volume, is set at a level not smaller than 60% of the numberof all the crystal grains of the alloy, it is possible to controlappropriately the fluctuation of the Mg content so as to suppress theuneven distribution of Mg, thereby further improving the reversibilityof the hydrogen absorption-desorption reaction. It follows that it ispossible to further improve the discharge capacity and thecharge-discharge cycle life of the secondary battery comprising thenegative electrode containing the particular hydrogen absorbing alloy.Also, it is possible to further improve the running performance such asthe fuel cost of the hybrid car or the electric automobile having theparticular secondary battery mounted thereto.

The fifth hydrogen absorbing alloy of the present invention has acomposition represented by general formula (4) given previously. Anamount of a phase having a AB₂ type crystal structure, which iscontained in the fifth hydrogen absorbing alloy of the presentinvention, is not larger than 10% by volume, including 0% by volume. Thefifth hydrogen absorbing alloy of the particular construction makes itpossible to improve the hydrogen desorption characteristics so as tofurther improve the reversibility of the hydrogen absorption-desorptionreaction and, thus, to increase the hydrogen storage capacity. Itfollows that it is possible to improve the discharge capacity and thecharge-discharge cycle life of the secondary battery comprising thenegative electrode containing the particular hydrogen absorbing alloy.Also, it is possible to further improve the running performance such asthe fuel cost of the hybrid car or the electric automobile having theparticular secondary battery mounted thereto.

The sixth hydrogen absorbing alloy of the present invention has acomposition represented by general formula (4) referred to above. Also,a parallel growth region precipitates in at least one crystal grain of aprincipal phase of the hydrogen absorbing alloy. The parallel growthregion has a crystal structure differing from a crystal structure of theprincipal phase. The sixth hydrogen absorbing alloy of the presentinvention having the particular construction makes it possible to lessenthe fluctuation in the Mg content so as to suppress an extremely unevendistribution of Mg and, thus, to improve the reversibility of thehydrogen absorption-desorption reaction. It follows that it is possibleto improve the discharge capacity and the charge-discharge cycle life ofthe secondary battery comprising the negative electrode containing theparticular hydrogen absorbing alloy. Also, it is possible to improve therunning performance such as the fuel cost of the hybrid car or theelectric automobile having the particular secondary battery mountedthereto.

It should be noted in particular that, if the number of crystal grains,in which the volume ratio of the parallel growth region is not higherthan 40% by volume, is set at a level not smaller than 60% of the numberof all the crystal grains of the alloy, it is possible to controlappropriately the fluctuation of the Mg content so as to suppress theuneven distribution of Mg, thereby further improving the reversibilityof the hydrogen absorption-desorption reaction. It follows that it ispossible to further improve the discharge capacity and thecharge-discharge cycle life of the secondary battery comprising thenegative electrode containing the particular hydrogen absorbing alloy.Also, it is possible to further improve the running performance such asthe fuel cost of the hybrid car or the electric automobile having theparticular secondary battery mounted thereto.

Some Examples of the present invention will now be described in detailwith reference to the accompanying drawings.

Table 1 shows the compositions of the mish metals Lm and Mm given inTables 2, 4 and 6 referred to herein later.

TABLE 1 (Lm, Mm shown in Tables 2, 4, 6 . . . unit: wt %) La Ce Pr Nd SmLm(1) 93 0.8 0.7 5.5 — Lm(2) 85 2.5 2.5 10 — Lm(3) 72 3.1 4.9 20 — Lm(4)48 3.5 14.5 34 — Mm(1) 38 50.2 3.5 8.1 0.2 Mm(2) 25 50 5.5 19.5 —

EXAMPLES 1 TO 16 AND COMPARATIVE EXAMPLES 1 TO 5

Hydrogen absorbing alloys were prepared by the high frequency inductionmelting method, the rapid solidification process or the mechanicalalloying method as described in the following.

(High Frequency Induction Melting Method)

Each of the elements constituting the composition shown in Table 2 wasweighed, followed by melting the composition in a high frequencyinduction furnace under an argon gas atmosphere so as to obtain an alloyingot. thus obtained under an argon gas atmosphere and under theconditions shown in Table 2 so as to obtain hydrogen absorbing alloysfor Examples 1 to 8, 13 to 16 and Comparative Examples 2, 4, 5.

(Rapid Solidification Process)

Each of the elements constituting the composition shown in Table 2 wasweighed, followed by melting the composition in a high frequencyinduction furnace under an argon gas atmosphere so as to obtain an alloyingot. The alloy ingot thus obtained was melted, followed by drippingthe resultant melt onto the surface of a single roll made of copper androtating at a peripheral velocity of 7 m/sec under an argon gasatmosphere so as to rapidly cool the melt and, thus, to obtain aflake-like hydrogen absorbing alloy. Further, a heat treatment wasapplied to the resultant hydrogen absorbing alloy flakes under an argongas atmosphere and under the conditions shown in Table 2 so as to obtainhydrogen absorbing alloys for Examples 9 to 12 and Comparative Example1.

(Mechanical Alloying Method)

A raw material metal mixture prepared by mixing La and Ni at an atomicratio La:Ni of 1:3 was melted in an arc furnace, followed by cooling themelt so as to obtain an alloy (a) consisting of a LaNi₃ phase. Also,another raw material metal mixture prepared by mixing La and Ni at anatomic ratio La:Ni of 1:4 was melted in an arc furnace, followed bycooling the melt so as to obtain an alloy (b) consisting of a LaNi₄phase. The alloy (a) in an amount of 100 g and the alloy (b) in anamount of 100 g were put in a planetary ball mill having steel balls puttherein so as to be subjected to a mechanical alloying for 10 hours atroom temperature under an argon gas atmosphere, thereby obtaining analloy (c) consisting of a LaNi₃ phase, a LaNi₄ phase and a La₅Ni₁₉phase.

On the other hand, a raw material metal mixture prepared by mixing La,Mg and Ni at an atomic ratio La:Mg:Ni of 0.9:0.1:5 was melted in an arcfurnace, followed by cooling the melt so as to obtain an alloy (d)consisting of a La_(0.9)Mg_(0.1)Ni₅ phase. The alloy (d) in an amount of30 g and the alloy (c) in an amount of 300 g were put in a planetaryball mill having steel balls put therein so as to be subjected to amechanical alloying for 10 hours at room temperature under an argon gasatmosphere, thereby obtaining an alloy for Comparative Example 3consisting of a LaNi₃ phase, a LaNi₄ phase, a La₅Ni₁₉ phase and aLa_(0.9)Mg_(0.1)Ni₅ phase.

Each of the hydrogen absorbing alloys thus prepared was pulverized underan inert gas atmosphere so as to permit the pulverized powder to have anaverage particle diameter of 55 μm. Then, 0.5 parts by weight of anickel powder available on the market, which had been prepared by thecarbonyl method, and 0.5 parts by weight of a Ketchen black powder wereadded to and mixed with 100 parts by weight of the alloy powder thusprepared. Further, a paste was prepared by adding 1 parts by weight ofpolytetrafluoro ethylene (PTFE), 0.2 parts by weight of sodiumpolyacrylate, 0.2 parts by weight of carboxymethyl cellulose, and 50parts by weight of water to 100 parts by weight of the mixed powder,followed by stirring the mixture. Still further, a punched thin ironplate having a nickel plating applied to the surface was coated with thepaste thus obtained, followed by drying the paste so as to obtain acoated plate. The thickness of the coated plate thus obtained wasadjusted by applying a roll press to the coated plate, followed bycutting the coated plate into pieces each having a desired size so as toprepare a negative electrode containing 7.5 g of the hydrogen absorbingalloy.

On the other hand, prepared as a separator was a polyolefin basedunwoven fabric having acrylic acid introduced therein by a graftcopolymerization.

An electrode group was prepared by spirally winding a laminate structurecomprising the negative electrode prepared as described above, a pastetype nickel positive electrode having a nominal capacity of 1,500 mAh,which had been prepared by the known method, and the separator describedabove, which was interposed between the positive electrode and thenegative electrode.

The electrode group thus prepared was put in a cylindrical case togetherwith 2.5 ml of an alkaline electrolyte containing 7 mol of KOH, 0.5 molof NaOH and 0.5 mol of LiOH, followed by sealing the case so as toassemble a cylindrical nickel hydrogen secondary battery of size AAhaving a nominal capacity of 1,500 mAh.

Each of the secondary batteries prepared in Examples 1 to 16 andComparative Examples 1 to 5 was left to stand at room temperature for 36hours. Then, a charge-discharge cycle, in which the secondary batterywas charged under a current of 150 mA for 15 hours, followed bydischarging the secondary battery under a current of 150 mA until thebattery voltage was lowered to 0.8V, was carried out twice. Further, thecharge-discharge cycle was repeated under an environment of 45° C. so asto measure the number of cycles reached at the time when the dischargecapacity of the secondary battery was lowered to 80% of the dischargecapacity of the first cycle. Table 3 shows the number of cycles and thedischarge capacity of the first cycle. Incidentally, the chargingprocess of the charge-discharge cycle was performed by employing a −ΔVmethod in which the charging current was set at 1,500 mA and thecharging was finished at the time when the voltage was lowered by 10 mVfrom the maximum voltage in the charging process. On the other hand, thedischarge process was performed under a current of 3,000 mA until thebattery voltage was lowered to 1.0V.

Also, characteristics (A) to (D) given below were measured in respect ofthe hydrogen absorbing alloy used in the secondary battery for each ofExamples 1 to 16 and Comparative Examples 1 to 5:

(A) A rechargeable hydrogen storage capacity (which is a term for ahydrogen absorbing alloy specified in JIS H7003) was obtained as ahydrogen absorption characteristic for each of the hydrogen absorbingalloys by measuring the iso-thermal line of pressure-composition at 60°C. and under a hydrogen pressure lower than 10 atm. by the Sievert'smethod (JIS H7201). Table 3 shows the results.

(B) The crystal structure of each of the hydrogen absorbing alloys wasobserved from the X-ray diffraction pattern by using a Cu—Kα ray as theX-ray source so as to determine the crystal structure of the principalphase, with the results as shown in Table 3. Incidentally, theexpression “Ce₂Ni₇ type+PuNi₃ type” in Table 3 denotes that theprincipal phase consists of two crystal phases, e.g., Ce₂Ni₇ type andPuNi₃ type.

FIG. 3 shows the X-ray diffraction patterns of the hydrogen absorbingalloys for Examples 1, 13 and 14. Diffraction pattern (a) shown in FIG.3 is the diffraction pattern of the hydrogen absorbing alloy for Example1 which contained as the principal phase the phase having the Ce₂Ni₇type crystal structure. In the hydrogen absorbing alloy for Example 1,main peak P appeared at the 2θ value (θ denoting the Bragg angle)falling within a range of 42.1°±1°, and main peak P1 appeared at the 2θvalue falling within a range of from 31° to 34°. Also, the peak Pappearing at the 2θ value falling within a range of 42.1°±1° has thehighest intensity. Diffraction patterns (b) and (c) shown in FIG. 2cover the hydrogen absorbing alloys for Examples 13 and 14,respectively, which contained as a principal phase a phase having acrystal structure similar to the Ce₂Ni₇ type structure. In diffractionpattern (b) shown in FIG. 2 (Example 13), peak P2 having the highestintensity appeared at the 2θ value falling within a range of 42.1°±1°and peak P3 appearing at the value of 2θ falling within a range of from31° to 34° was found as being split into three. Also, the intensity ofpeak P3 was lower than that of peak P1. On the other hand, indiffraction pattern (c) shown in FIG. 2 (Example 14), peak P4 having thehighest intensity appeared at the 2θ value falling within a range of42.1°±1° and peak P5 having the intensity lower than that of peak P1appeared at the value of 2θ falling within a range of from 31° to 34°.Incidentally, the intensity ratio of the diffraction pattern (c), whichwas calculated by using formula (I) given below, was found to be 22%:I₃/I₄  (I)

where I₄ denotes the intensity of peak P4, and I₃ denotes the intensityof peak P5.

Further, the electron diffraction analysis pattern was photographed by atransmission electron microscope (TEM) in respect of the hydrogenabsorbing alloy for each of Examples 1, 13 and 14. FIG. 4 is themicrograph showing the electron diffraction analysis pattern of thehydrogen absorbing alloy for Example 14. Also, FIG. 5 is a schematicdrawing for explaining the micrograph shown in FIG. 4. As apparent fromFIGS. 4 and 5, there are four supper lattice reflection spots betweenthe primitive lattice reflection spot (00L) and the origin (000) in thehydrogen absorbing alloy for Example 14. It is also seen that thesesupper lattice reflection spots are present at four points at which thedistance |G_(00L)| between the primitive lattice reflection spot (00L)and the origin (000) is equally divided into five sections.Incidentally, the distance |G_(00L)| was found to be 0.4 nm⁻¹. It hasalso been found that the supper lattice reflection spots are present atfour points at which the distance |G_(00L)| between the primitivelattice reflection spot (00L) and the origin (000) is equally dividedinto five sections in the electron diffraction analysis pattern for thehydrogen absorbing alloy for Example 13. On the other hand, it has beenfound that the supper lattice reflection spots are present at two pointsat which the distance |G_(00L)| between the primitive lattice reflectionspot (00L) and the origin (000) is equally divided into three sectionsin the electron diffraction analysis pattern for the hydrogen absorbingalloy for Example 1.

Incidentally, the principal phase of the hydrogen absorbing alloy forExample 8 contains a phase having a PuNi₃ type structure and anotherphase having a Ce₂Ni₇ structure. In the hydrogen absorbing alloy forExample 8, the peaks appearing respectively at the 2θ value fallingwithin a range of 42.1°±1° and at the 2θ value falling within a range offrom 31° to 34° are the main peaks. Also, the peak appearing at the 2θvalue falling within a range of 42.1°±1° was found to have the highestintensity. The hydrogen absorbing alloy for each of Examples 15 and 16contains as the principal phase a phase having a crystal structuresimilar to the PuNi₃ type structure. In the hydrogen absorbing alloy forExample 15, the peak having the highest intensity appeared at the valueof 2θ falling within a range of 42.1°±1°, and the peak appearing at thevalue of 2θ falling within a range of from 31° to 34° was found as beingsplit into two. Also, the intensity of the peak split into two was lowerthan that of the peak that appeared at the value of 2θ falling within arange of from 31° to 34° in the diffraction pattern for Example 8referred to previously. On the other hand, the peak having the highestintensity appeared at the value of 2θ falling within a range of 42.1°±1°in the X-ray diffraction pattern of the hydrogen absorbing alloy forExample 16, and the intensity of the peak appearing at the value of 2θfalling within a range of from 31° to 34° was lower than that of thepeak that appeared at the value of 2θ falling within a range of from 31°to 34° in the diffraction pattern for Example 8 referred to previously.The intensity ratio calculated by formula (I) referred to previously wasfound to be 18% in the hydrogen absorbing alloy for Example 16.

(C) A secondary electron image and a reflected electron image of each ofthe hydrogen absorbing alloys was photographed by using a scanningelectron microscope (SEM) so as to detect a phase differing from theprincipal phase. The composition of the phase differing from theprincipal phase was analyzed by an energy dispersive X-ray spectroscopy(EDX) of the scanning electron microscope. It has been found from theresult of the composition analysis and the X-ray diffraction patternobtained in item (B) referred to above that the phase differing from theprincipal phase was a phase having a MgCu₂ type crystal structure.

(D) The contents of the principal phase and the MgCu₂ type phase weremeasured as follows for each of the hydrogen absorbing alloys so as toobtain the results as shown in Table 3.

Optional five view fields of the hydrogen absorbing alloy for each ofExamples 1 to 8, 13 to 16 and Comparative Examples 2, 4, 5 werephotographed by a scanning electron microscope. The area ratio of thetarget phase based on the alloy area within the view field was obtainedfor each of the micrographs. The average value of the area ratios thusobtained was calculated and given in Table 3 as the volume ratio of thetarget phase in the hydrogen absorbing alloy.

On the other hand, optional five view fields of the hydrogen absorbingalloy for each of Examples 9 to 12 and Comparative Examples 1, 3 werephotographed by a transmission electron microscope. The area ratio ofthe target phase based on the alloy area within the view field wasobtained for each of the micrographs. The average value of the arearatios thus obtained was calculated and given in Table 3 as the volumeratio of the target phase in the hydrogen absorbing alloy.

TABLE 2 Heat treatment conditions Temperature Time Composition (° C.)(h) Example 1 Lm(2)_(0.77)Mg_(0.23)Ni_(3.34)Al_(0.08) 935 10  Example 2La_(0.6)Nd_(0.14)Mg_(0.26)Ni_(3.29)Mn_(0.02)Si_(0.01) 940 8 Example 3La_(0.53)Pr_(0.2)Mg_(0.27)Ni_(3.1)Co_(0.1)Fe_(0.01)Al_(0.08) 945 6Example 4 Lm(2)_(0.77)Mg_(0.23)Ni_(3.35)Al_(0.1)Mo_(0.005) 940 5 Example5 Lm(1)_(0.8)Mg_(0.2)Ni_(3.4)Co_(0.05)Mn_(0.02)Al_(0.1)Ta_(0.005) 915 7Example 6La_(0.75)Ce_(0.05)Mg_(0.2)Ni_(3.45)Zn_(0.02)Al_(0.12)Li_(0.005) 920 6Example 7 Lm(1)_(0.74)Y_(0.07)Mg_(0.19)Ni_(3.45)Co_(0.08)Sn_(0.01) 925 9Example 8 Lm(3)_(0.7)Mg_(0.3)Ni_(3.25)Cu_(0.02)Al_(0.06) 945 7 Example 9Lm(4)_(0.76)Mg_(0.24)Ni_(3.35)Co_(0.02)W_(0.002)B_(0.01)Al_(0.05) 970 2Example 10Lm(1)_(0.68)Mm(1)_(0.11)Mg_(0.21)Ni_(3.14)Co_(0.2)Al_(0.12)Ga_(0.03) 9301 Example 11Lm(1)_(0.66)Mm(2)_(0.1)Mg_(0.24)Ni_(3.3)Co_(0.06)Mn_(0.03)V_(0.002)Al_(0.07)935 3 Example 12Lm(4)_(0.76)Ca_(0.01)Mg_(0.23)Ni_(3.3)Cr_(0.002)Al_(0.11) 980 2 Example13 Lm(2)_(0.765)Zr_(0.005)Mg_(0.23)Ni_(3.31)P_(0.002)Al_(0.1) 935 4Example 14 Lm(3)_(0.775)Ti_(0.005)Mg_(0.22)Ni_(3.36)Al_(0.15)S_(0.003)945 3 Example 15 Lm(3)_(0.77)Mg_(0.23)Ni_(3.06)Co_(0.13)Al_(0.12) 940 5Example 16Lm(1)_(0.73)Mm(1)_(0.05)Mg_(0.22)Ni_(3.13)Co_(0.2)Mn_(0.01)Sn_(0.02)Al_(0.07)960 4 ComparativeMm(2)_(0.65)Mg_(0.35)Ni_(2.27)Co_(0.3)Mn_(0.3)Fe_(0.02)Al_(0.14) 850 4Example 1 Comparative(La_(0.32)Ce_(0.48)Pr_(0.05)Nd_(0.13)Sm_(0.02))_(0.975)Mg_(0.025)Ni_(2.925)Mn_(0.35)Cu_(0.1)Nb_(0.075)900 12  Example 2 Comparative La_(0.917)Mg_(0.083)Ni_(4.75) — — Example3 Comparative La_(0.9)Mg_(0.1)Ni_(4.8) 1000  4 Example 4 ComparativeLa_(0.5)Mg_(0.5)Ni_(2.1) 800 5 Example 5 Note: The alloys for Examples 9to 12 and Comparative Example 1 were prepared by a rapid solidificationprocess, and the alloy for Comparative Example 3 was prepared by amechanical alloying method.

TABLE 3 Content Rechargeable Principal of AB₂ hydrogen phase typestorage content phase Capacity Cycle capacity Crystal structure of (% by(% by (mAh) life (H/M) principal phase volume) volume) Example 1 1350325 1.09 Ce₂Ni₇ type 98 0 Example 2 1335 320 1.08 Ce₂Ni₇ type + PuNi₃type 96 0.5 Example 3 1360 295 1.06 Ce₂Ni₇ type + CeNi₃ type 92 1.1Example 4 1355 290 1.05 Ce₂Ni₇ type + PuNi₃ type 91 1.5 Example 5 1350295 1.07 Ce₂Ni₇ type 94 0.8 Example 6 1320 285 1.03 Ce₂Ni₇ type 89 2.1Example 7 1330 280 0.92 Ce₂Ni₇ type 88 2.6 Example 8 1368 285 1.08Ce₂Ni₇ type + PuNi₃ type 90 2.4 Example 9 1365 270 0.94 Ce₂Ni₇ type 883.6 Example 10 1335 265 0.96 Ce₂Ni₇ type + Gd₂Co₇ type 85 4.8 Example 111355 320 0.99 Ce₂Ni₇ type 94 0.4 Example 12 1345 230 0.97 Ce₂Ni₇ type 901.2 Example 13 1335 295 0.89 Similar to Ce₂Ni₇ type 89 1.1 Example 141340 275 0.87 Similar to Ce₂Ni₇ type 82 3.5 Example 15 1330 250 0.86Similar to PuNi₃ type 83 4.2 Example 16 1325 220 0.85 Similar to PuNi₃type 72 5.3 Comparative 1290 70 0.68 CaCu₅ type 80 15 Example 1Comparative 1020 25 0.57 Ce₂Ni₇ type 63 18 Example 2 Comparative 700 100.45 CaCu₅ type 92 0 Example 3 Comparative 1250 25 0.69 CaCu₅ type 95 3Example 4 Comparative 800 15 0.42 MgCu₂ type 75 — Example 5

To reiterate, the hydrogen absorbing alloy for each of Examples 1 to 16contains as the principal phase at least one kind of the phase selectedfrom the group consisting of the first phase referred to previously andthe second phase referred to previously, and the amount of the AB₂ typephase such as the MgCu₂ type phase contained in the hydrogen absorbingalloy is not larger than 10% by volume. As apparent from Tables 2 and 3,the rechargeable hydrogen storage capacity of the hydrogen absorbingalloy for each of Examples 1 to 16 is larger than that of the hydrogenabsorbing alloy for each of Comparative Examples 1 to 5. Incidentally,the hydrogen absorbing alloy for Comparative Example 1 has a compositionequal to that of the hydrogen absorbing alloy disclosed in U.S. Pat. No.5,840,166 and contains the CaCu₅ type phase as the principal phase.Also, the hydrogen absorbing alloy for Comparative Example 2 has acomposition equal to that of the hydrogen absorbing alloy disclosed inJapanese Patent Disclosure No. 11-29832, contains the Ce₂Ni₇ type phaseas the principal phase, and contains an AB₂ type phase in an amountexceeding 10% by volume. On the other hand, the hydrogen absorbing alloyfor Comparative Example 3 has a composition equal to that of thehydrogen absorbing alloy disclosed in Japanese Patent Disclosure No.10-1731.

It is also seen that the secondary battery for each of Examples 1 to 16was superior to the secondary battery for each of Comparative Examples 1to 5 in each of the discharge capacity and the charge-discharge cyclelife.

EXAMPLES 17 TO 32 AND COMPARATIVE EXAMPLES 6 TO 10

Hydrogen absorbing alloys were prepared by the high frequency inductionmelting method, the rapid solidification process or the mechanicalalloying method as described in the following.

(High Frequency Induction Melting Method)

Each of the elements constituting the composition shown in Table 4 wasweighed, followed by melting the composition in a high frequencyinduction furnace under an argon gas atmosphere so as to obtain an alloyingot. Then, a heat treatment was applied to the alloy ingot thusobtained under an argon gas atmosphere and under the conditions shown inTable 4 so as to obtain hydrogen absorbing alloys for Examples 17 to 25and 29 to 32 and Comparative Examples 7, 9, 10.

(Rapid Solidification Process)

Each of the elements constituting the composition shown in Table 4 wasweighed, followed by melting the composition in a high frequencyinduction furnace under an argon gas atmosphere so as to obtain an alloyingot. The alloy ingot thus obtained was melted, followed by drippingthe resultant melt onto the surface of a single roll made of copper androtating at a peripheral velocity of 10 m/sec under an argon gasatmosphere so as to rapidly cool the melt and, thus, to obtain aflake-like hydrogen absorbing alloy. Further, a heat treatment wasapplied to the resultant hydrogen absorbing alloy flakes under an argongas atmosphere and under the conditions shown in Table 4 so as to obtainhydrogen absorbing alloys for Examples 26 to 28 and Comparative Example6.

(Mechanical Alloying Method)

A raw material metal mixture prepared by mixing Lm(1) and Ni at anatomic ratio Lm(1):Ni of 1:3 was melted in an arc furnace, followed bycooling the melt so as to obtain an alloy (a) consisting of a Lm(1)Ni₃phase. Also, another raw material metal mixture prepared by mixing Lm(1)and Ni at an atomic ratio Lm(1):Ni of 1:4 was melted in an arc furnace,followed by cooling the melt so as to obtain an alloy (b) consisting ofa Lm(1)Ni₄ phase. The alloy (a) in an amount of 100 g and the alloy (b)in an amount of 100 g were put in a planetary ball mill having steelballs put therein so as to be subjected to a mechanical alloying for 10hours at room temperature under an argon gas atmosphere, therebyobtaining an alloy (c) consisting of a Lm(1)Ni₃ phase, a Lm(1)Ni₄ phaseand a Lm(1)₅Ni₁₉ phase.

On the other hand, a raw material metal mixture prepared by mixingLm(1), Mg and Ni at an atomic ratio Lm(1):Mg:Ni of 0.9:0.1:5 was meltedin an arc furnace, followed by cooling the melt so as to obtain an alloy(d) consisting of a Lm(1)_(0.9)Mg_(0.1)Ni₅ phase. The alloy (d) in anamount of 30 g and the alloy (c) in an amount of 300 g were put in aplanetary ball mill having steel balls put therein so as to be subjectedto a mechanical alloying for 10 hours at room temperature under an argongas atmosphere, thereby obtaining an alloy for Comparative Example 8consisting of a Lm(1)Ni₃ phase, a Lm(1)Ni₄ phase, a Lm(1)₅Ni₁₉ phase anda Lm(1)_(0.9)Mg_(0.1)Ni₅ phase.

A cylindrical nickel hydrogen secondary battery was assembled as inExample 1 by using each of the hydrogen absorbing alloys thus prepared.

The secondary battery prepared in each of Examples 17 to 32 andComparative Examples 6 to 10 was left to stand under room temperaturefor 36 hours. Then, each of the discharge capacity and thecharge-discharge cycle life of the secondary battery were measured as inExample 1 so as to obtain the results shown in Table 5.

Also, characteristics (A) to (F) given below were measured in respect ofthe hydrogen absorbing alloy used in the secondary battery for each ofExamples 17 to 32 and Comparative Examples 6 to 10:

(A) A rechargeable hydrogen storage capacity was obtained for each ofthe hydrogen absorbing alloys as in Example 1. Table 5 shows theresults.

(B) The crystal structure of each of the hydrogen absorbing alloys wasobserved from the X-ray diffraction pattern by using a Cu—Kα ray as theX-ray source so as to determine the crystal structure of the principalphase. Table 5 shows the results.

In the hydrogen absorbing alloy for Example 29, the peak having thehighest intensity appeared at the value of 2θ falling within a range of42.1°±1° in the X-ray diffraction pattern, and a peak appeared at thevalue of 2θ falling within a range of from 31° to 34° was found as beingsplit into two. The intensity of the two-split peak was lower than thatof peak P1 in diffraction pattern (a) of the normal structure referredto previously. On the other hand, in the hydrogen absorbing alloy forExample 30, the peak having the highest intensity appeared at the valueof 2θ falling within a range of 42.1°±1° in the X-ray diffractionpattern, and a peak having an intensity lower than that of peak P1referred previously appeared at the value of 2θ falling within a rangeof from 31° to 34°. Incidentally, the intensity ratio calculated byformula (I) referred to previously on the basis of the diffractionpattern of the hydrogen absorbing alloy for Example 30 was found to be24%.

Further, the electron diffraction analysis pattern for the hydrogenabsorbing alloy for each of Examples 29 and 30 was photographed by atransmission electron microscope (TEM). It has been found that therewere four supper lattice reflection spots between the primitive latticereflection points (00L) and the origin (000) in the hydrogen absorbingalloy for each of Examples 29 and 30. It has also been found that thesupper lattice reflection spots were present in four points at which thedistance |G_(00L)| between the primitive lattice reflection spot (00L)and the origin (000) was equally divided into five sections.

Also, the peak having the highest intensity appeared at the value of 2θfalling within a range of 42.1°±1° in the X-ray diffraction pattern ofthe hydrogen absorbing alloy for Example 31. Further, a peak appeared atthe value of 2θ falling within a range of from 31° to 34° was found asbeing split into three. The intensity of the three-split peak was foundto be lower than that of the peak appearing in the diffraction patternof the normal structure referred to previously. On the other hand, thepeak having the highest intensity appeared at the value of 2θ fallingwithin a range of 42.1°±1° in the X-ray diffraction pattern of thehydrogen absorbing alloy for Example 32. Also, a peak having anintensity lower than that of peak of the normal structure referred topreviously appeared at the value of 2θ falling within a range of from31° to 34°. Incidentally, the intensity ratio calculated by formula (I)referred to previously on the basis of the diffraction pattern of thehydrogen absorbing alloy for Example 32 was found to be 19%.

(C) The volume ratio of the principal phase for each of the hydrogenabsorbing alloys was measured as follows. Table 5 shows the results.

Specifically, optional five view fields of the hydrogen absorbing alloyfor each of Examples 17 to 25, 29 to 32 and Comparative Examples 7, 9,10 were photographed by a scanning electron microscope. Then, the arearatio of the target phase relative to the alloy area within the viewfield was obtained for each of the micrographs. Further, the averagevalue of the area ratios thus obtained was calculated and given in Table5 as the volume ratio of the target phase in the hydrogen absorbingalloy.

On the other hand, optional five view fields of the hydrogen absorbingalloy for each of Examples 26 to 28 and Comparative Examples 6, 8 werephotographed by a transmission electron microscope. Then, the area ratioof the target phase relative to the alloy area within the view field wasobtained for each of the micrographs. Further, the average value of thearea ratios thus obtained was calculated and given in Table 5 as thevolume ratio of the target phase in the hydrogen absorbing alloy.

(D) (1,0,0) plane of the crystal grain in optional 30 view fields werephotographed by a transmission electron microscope with a magnificationof 30,000 in respect of each of the hydrogen absorbing alloys. For eachview field, the area of the parallel growth region excluding theprincipal phase was measured. Then the area ratio of the parallel growthregion relative to the alloy area within the view field was calculated.The average value of the calculated area ratios of the 30 view fieldswas obtained and given in Table 5 as the volume ratio of the parallelgrowth region of the crystal grain.

(E) (1,0,0) plane of the crystal grain in optional 30 view fields werephotographed by a transmission electron microscope with a magnificationof 30,000 in respect of each of the hydrogen absorbing alloys. For eachview field, the area of the parallel growth region excluding theprincipal phase was measured. The area ratio of the parallel growthregion relative to the alloy area within the view field was calculated.Further, a ratio of the number of view fields, in which the volume ratioof the parallel growth region was not higher than 40% by volume, to the30 view fields was calculated and given in Table 5 as a ratio of thenumber of crystal grains, in which the volume ratio of the parallelgrowth region was not higher than 40% by volume, to the total number ofcrystal grains, said ratio being hereinafter referred to as a “crystalgrain ratio”.

(F) The composition of the parallel growth region excluding theprincipal phase of each of the hydrogen absorbing alloys was analyzed byan energy dispersive X-ray spectroscopy (EDX) of the scanning electronmicroscope. The crystal structure of the parallel growth was specifiedfrom the result of the composition analysis and the X-ray diffractionpattern obtained in item (B) described previously. The results are shownin Table 5.

FIG. 6 shows one view field of the hydrogen absorbing alloy for Example23, which was selected from among the micrographs obtained inmeasurements (D) and (E) described previously. The grayish obliquepattern present in the central portion of the photo of FIG. 6 denotesthe parallel growth region excluding the principal phase. Incidentally,the curved pattern shown in the photo denotes an interference fringes.

TABLE 4 Heat treatment conditions Temperature Time Composition (° C.)(h) Example 17 Lm(1)_(0.77)Mg_(0.23)Ni_(3.34)Al_(0.11) 925 6 Example 18La_(0.62)Pr_(0.14)Mg_(0.24)Ni_(3.29)Mn_(0.02)Fe_(0.01) 950 8 Example 19La_(0.55)Nd_(0.2)Mg_(0.25)Ni_(3.1)Co_(0.1)Si_(0.01)Al_(0.08) 970 10 Example 20 Lm(3)_(0.77)Mg_(0.23)Ni_(3.35)Al_(0.1)Ta_(0.005) 940 5Example 21Lm(2)_(0.8)Mg_(0.2)Ni_(3.4)Co_(0.05)Mn_(0.02)Al_(0.1)Mo_(0.005) 935 8Example 22La_(0.71)Ce_(0.07)Mg_(0.22)Ni_(3.35)Zn_(0.03)Al_(0.12)Li_(0.003) 920 5Example 23 Lm(2)_(0.74)Y_(0.07)Mg_(0.19)Ni_(3.45)Co_(0.1)Sn_(0.01) 930 4Example 24 Lm(1)_(0.71)Mg_(0.29)Ni_(3.25)Cu_(0.02)Al_(0.08) 915 7Example 25Lm(3)_(0.76)Mg_(0.24)Ni_(3.35)Co_(0.02)W_(0.002)B_(0.005)Al_(0.05) 960 8Example 26 Lm(2)_(0.7)Mm(1)_(0.09)Mg_(0.21)Ni_(3.32)Al_(0.12)Ga_(0.03)950 9 Example 27Lm(1)_(0.71)Mm(2)_(0.05)Mg_(0.24)Ni_(3.3)Co_(0.06)Mn_(0.03)V_(0.002)Al_(0.07)965 5 Example 28Lm(4)_(0.76)Ca_(0.01)Mg_(0.23)Ni_(3.3)Cr_(0.003)Al_(0.14) 980 2 Example29 Lm(1)_(0.755)Zr_(0.005)Mg_(0.24)Ni_(3.31)P_(0.002)Al_(0.1) 955 10 Example 30 Lm(2)_(0.785)Ti_(0.005)Mg_(0.21)Ni_(3.36)Al_(0.15)S_(0.003)965 8 Example 31 Lm(2)_(0.76)Mg_(0.24)Ni_(3.18)Co_(0.15)Al_(0.12) 950 7Example 32Lm(1)_(0.73)Mm_(0.05)Mg_(0.22)Ni_(3.02)Co_(0.2)Mn_(0.01)Sn_(0.02)Al_(0.07)960 5 ComparativeMm(2)_(0.72)Mg_(0.28)Ni_(2.66)Co_(0.4)Mn_(0.4)Fe_(0.02)Al_(0.15) 900 4Example 6 Comparative(La_(0.3)Ce_(0.49)Pr_(0.05)Nd_(0.14)Sm_(0.02))_(0.975)Mg_(0.025)Ni₃Mn_(0.35)Cu_(0.1)Ga_(0.05)900 12  Example 7 Comparative Lm(1)_(0.917)Mg_(0.083)Ni_(4.75) — —Example 8 Comparative La_(0.9)Mg_(0.1)Ni_(4.6) 1000  5 Example 9Comparative La_(0.55)Mg_(0.45)Ni_(2.3) 800 7 Example 10 Note: The alloysfor Examples 26 to 28 and Comparative Example 6 were prepared by a rapidsolidification process, and the alloy for Comparative Example 8 wasprepared by a mechanical alloying method.

TABLE 5 Parallel growth Rechargeable Principal content of hydrogenCrystal phase crystal Crystal Crystal storage structure of content graingrain structure of Capacity Cycle capacity principal (% by (% by ratioparallel (mAh) life (H/M) phase volume) volume) (%) growth Example 171350 325 1.08 Ce₂Ni₇ type 99 5 95 PuNi₃ type Example 18 1320 315 1.05Ce₂Ni₇ type + PuNi₃ 97 7 92 PuNi₃ type + A₅B₁₉ type type Example 19 1330290 1.04 Ce₂Ni₇ type + PuNi₃ 93 15 88 CeNi₃ type + A₅B₁₉ type typeExample 20 1368 280 1.06 Ce₂Ni₇ type + CeNi₃ 91 17 85 PuNi₃ type + A₅B₁₉type type Example 21 1365 295 1.04 Ce₂Ni₇ type 94 8 90 PuNi₃ typeExample 22 1335 285 1.03 Ce₂Ni₇ type 93 14 82 A₅B₁₉ type Example 23 1355280 1.02 Ce₂Ni₇ type 95 18 83 PuNi₃ type + A₅B₁₉ type Example 24 1350285 0.98 Ce₂Ni₇ type + PuNi₃ 96 20 85 CeNi₃ type + A₅B₁₉ type typeExample 25 1335 270 1.02 Ce₂Ni₇ type 92 24 72 PuNi₃ type + A₅B₁₉ typeExample 26 1360 265 0.96 Ce₂Ni₇ type + Gd₂Co₇ 90 30 70 CeNi₃ type +A₅B₁₉ type type Example 27 1355 320 0.99 Ce₂Ni₇ type 94 4 90 PuNi₃type + A₅B₁₉ type Example 28 1365 235 0.97 Ce₂Ni₇ type 90 8 88 PuNi₃type + A₅B₁₉ type Example 29 1335 295 0.88 Similar to 91 10 75 PuNi₃type + A₅B₁₉ Ce₂Ni₇ type type Example 30 1355 275 0.86 Similar to 88 1880 PuNi₃ type + A₅B₁₉ Ce₂Ni₇ type type Example 31 1345 250 0.84 Similarto 85 35 65 Ce₂Ni₇ type + A₅B₁₉ PuNi₃ type type Example 32 1350 220 0.83Similar to 78 40 60 Ce₂Ni₇ type + A₅B₁₉ PuNi₃ type type Comparative 125070 0.68 CaCu₅ type 82 55 55 Ce₂Ni₇ type Example 6 Comparative 1030 250.57 Ce₂Ni₇ type 65 50 50 CaCu₅ type + PuNi₃ Example 7 type Comparative740 10 0.45 CaCu₅ type 93 3 88 Ce₂Ni₇ type + A₅B₁₉ Example 8 typeComparative 1200 15 0.69 CaCu₅ type 92 15 78 Ce₂Ni₇ type + A₅B₁₉ Example9 type Comparative 750 15 0.42 MgCu₂ type 72 30 45 PuNi₃ type Example 10

To reiterate, the hydrogen absorbing alloy for each of Examples 17 to 32contains as the principal phase at least one kind of the phase selectedfrom the group consisting of the first phase referred to previously andthe second phase referred to previously, and has a compositionrepresented by formula (1) given previously. Also, a parallel growthregion that has a crystal structure differing from the crystal structureof the principal phase precipitates in at least one crystal grain of theprincipal phase in the hydrogen absorbing alloy for each of Examples 17to 32. As apparent from Tables 4 and 5, the rechargeable hydrogenstorage capacity of the hydrogen absorbing alloy for each of Examples 17to 32 is larger than that of the hydrogen absorbing alloy for each ofComparative Examples 6 to 10. Incidentally, the hydrogen absorbing alloyfor Comparative Example 6 has a composition equal to that of thehydrogen absorbing alloy disclosed in U.S. Pat. No. 5,840,166 andcontains the CaCu₅ type phase as the principal phase. Also, the hydrogenabsorbing alloy for Comparative Example 7 has a composition equal tothat of the hydrogen absorbing alloy disclosed in Japanese PatentDisclosure No. 11-29832, and contains as the principal phase the Ce₂Ni₇type phase. On the other hand, the hydrogen absorbing alloy forComparative Example 8 has a composition equal to that of the hydrogenabsorbing alloy disclosed in Japanese Patent Disclosure No. 10-1731.

It is also seen that the secondary battery for each of Examples 17 to 32was superior to the secondary battery for each of Comparative Examples 6to 10 in each of the discharge capacity and the charge-discharge cyclelife.

EXAMPLES 33 TO 40

Hydrogen absorbing alloys were prepared by the high frequency inductionmelting method as described in the following.

(High Frequency Induction Melting Method)

Each of the elements constituting the composition shown in Table 6 wasweighed, followed by melting the composition in a high frequencyinduction furnace under an argon gas atmosphere so as to obtain an alloyingot. Then, a heat treatment was applied to the alloy ingot thusobtained under an argon gas atmosphere and under the conditions shown inTable 6 so as to obtain hydrogen absorbing alloys for Examples 33 to 40.

A cylindrical nickel hydrogen secondary battery was assembled as inExample 1 by using each of the hydrogen absorbing alloys thus prepared.

The secondary battery prepared in each of Examples 33 to 40 was left tostand under room temperature for 36 hours. Then, each of the dischargecapacity and the charge-discharge cycle life of the secondary batterywere measured as in Example 1 so as to obtain the results shown in Table6.

The rechargeable hydrogen storage capacity, the crystal structure andthe content of the principal phase, the content of the AB₂ type phase,the content of the parallel growth in the crystal grain, the crystalgrain ratio, and the crystal structure of the parallel growth weremeasured as in Examples 1 and 17 in respect of the hydrogen absorbingalloy used in the secondary battery for each of Examples 33 to 40.Tables 6 and 7 show the results.

TABLE 6 Rechargeable Heat treatment hydrogen conditions storageTemperature Time Capacity Cycle capacity Composition (° C.) (h) (mAh)life (H/M) Example 33 Lm(1)_(0.76)Mg_(0.24)Ni_(3.32)Al_(0.11) 910 7 1350325 1.04 Example 34 Lm(2)_(0.77)Mg_(0.23)Ni_(3.24)Co_(0.05)Al_(0.11) 9305 1340 330 1.05 Example 35Lm(3)_(0.77)Mg_(0.23)Ni_(3.24)Mn_(0.02)Al_(0.13) 950 7 1345 320 1.04Example 36 Lm(3)_(0.79)Mg_(0.21)Ni_(3.3)Mn_(0.08)Al_(0.12) 955 4 1360340 1.06 Example 37Lm(4)_(0.78)Mg_(0.22)Ni_(3.15)Co_(0.1)Mn_(0.03)Al_(0.15) 960 8 1365 3051.04 Example 38 Lm(3)_(0.77)Mg_(0.23)Ni_(3.24)Cu_(0.03)Al_(0.11) 955 101350 335 1.03 Example 39 Lm(2)_(0.77)Mg_(0.23)Ni_(3.3)Al_(0.13) 935 61365 340 1.03 Example 40 Lm(3)_(0.76)Mg_(0.24)Ni_(3.3)Al_(0.12) 940 71360 345 1.05

TABLE 7 Principal Parallel Crystal phase Content of growth contentCrystal Crystal structure of content AB₂ type of crystal grain structureof principal (% by phase (% grain ratio parallel phase volume) byvolume) (% by volume) (%) growth Example 33 Ce₂Ni₇ type 98 0 5 95 PuNi₃type + A₅B₁₉ type Example 34 Ce₂Ni₇ type + PuNi₃ 96 0.5 7 92 PuNi₃type + A₅B₁₉ type type Example 35 Ce₂Ni₇ type 92 1.1 12 88 CeNi₃ type +A₅B₁₉ type Example 36 Ce₂Ni₇ type 95 1.5 11 90 PuNi₃ type + A₅B₁₉ typeExample 37 Ce₂Ni₇ type 94 0.8 8 90 PuNi₃ type + A₅B₁₉ type Example 38Ce₂Ni₇ type + PuNi₃ 97 0.4 6 92 CeNi₃ type + A₅B₁₉ type type Example 39Ce₂Ni₇ type 96 0.7 7 93 PuNi₃ type + A₅B₁₉ type Example 40 Ce₂Ni₇ type94 0.9 8 95 PuNi₃ type + A₅B₁₉ type

As apparent from Tables 6 and 7, the secondary battery for each ofExamples 33 to 40 has a long charge-discharge cycle life, which exceeds300.

Table 8 shows the compositions of mish metals Lm and Mm referred toherein later in Tables 9, 11 and 13:

TABLE 8 (Lm, Mm shown in Tables 9, 11, 13 . . . unit: wt %) La Ce Pr NdSm Lm(5) 95 0.4 1.2 3.4 — Lm(6) 83 1.3 2.3 13.4 — Lm(7) 71 2.2 5.6 21.2— Lm(8) 50 2.8 15.7 31.5 — Mm(3) 35 53 1.8 9.9 0.3

EXAMPLES 41 TO 56 AND COMPARATIVE EXAMPLES 11 TO 15

Hydrogen absorbing alloys were prepared by the high frequency inductionmelting method, the rapid solidification process or the mechanicalalloying method as described in the following.

(High Frequency Induction Melting Method)

Each of the elements constituting the composition shown in Table 9 wasweighed, followed by melting the composition in a high frequencyinduction furnace under an argon gas atmosphere so as to obtain an alloyingot. Then, a heat treatment was applied to the alloy ingot thusobtained under an argon gas atmosphere and under the conditions shown inTable 9 so as to obtain hydrogen absorbing alloys for Examples 41 to 48,53 to 56 and Comparative Examples 12, 14, 15.

(Rapid Solidification Process)

Each of the elements constituting the composition shown in Table 9 wasweighed, followed by melting the composition in a high frequencyinduction furnace under an argon gas atmosphere so as to obtain an alloyingot. The alloy ingot thus obtained was melted, followed by drippingthe resultant melt onto the surface of a single roll made of copper androtating at a peripheral velocity of 5 m/sec under an argon gasatmosphere so as to rapidly cool the melt and, thus, to obtain aflake-like hydrogen absorbing alloy. Further, a heat treatment wasapplied to the resultant hydrogen absorbing alloy flakes under an argongas atmosphere and under the conditions shown in Table 9 so as to obtainhydrogen absorbing alloys for Examples 49 to 52 and Comparative Example11.

(Mechanical Alloying Method)

A raw material metal mixture prepared by mixing Lm(5) and Ni at anatomic ratio Lm(5):Ni of 1:3 was melted in an arc furnace, followed bycooling the melt so as to obtain an alloy (a) consisting of a Lm(5)Ni₃phase. Also, another raw material metal mixture prepared by mixing Lm(5)and Ni at an atomic ratio Lm(5):Ni of 1:4 was melted in an arc furnace,followed by cooling the melt so as to obtain an alloy (b) consisting ofa Lm(5)Ni₄ phase. The alloy (a) in an amount of 100 g and the alloy (b)in an amount of 100 g were put in a planetary ball mill having steelballs put therein so as to be subjected to a mechanical alloying for 10hours at room temperature under an argon gas atmosphere, therebyobtaining an alloy (c) consisting of a Lm(5)Ni₃ phase, a Lm(5)Ni₄ phaseand a Lm(5)₅Ni₁₉ phase.

On the other hand, a raw material metal mixture prepared by mixingLm(5), Mg and Ni at an atomic ratio Lm(5):Mg:Ni of 0.9:0.1:5 was meltedin an arc furnace, followed by cooling the melt so as to obtain an alloy(d) consisting of a Lm(5)_(0.9)Mg_(0.1)Ni₅ phase. The alloy (d) in anamount of 30 g and the alloy (c) in an amount of 300 g were put in aplanetary ball mill having steel balls put therein so as to be subjectedto a mechanical alloying for 10 hours at room temperature under an argongas atmosphere, thereby obtaining an alloy for Comparative Example 13consisting of a Lm(5)Ni₃ phase, a Lm(5)Ni₄ phase, a Lm(5)₅Ni₁₉ phase anda Lm(5)_(0.9)Mg_(0.1)Ni₅ phase.

Each of the hydrogen absorbing alloys thus prepared was pulverized underan inert gas atmosphere so as to permit the pulverized powder to have anaverage particle diameter of 50 μm. Then, 0.5 parts by weight of anickel powder available on the market, which had been prepared by thecarbonyl method, and 0.5 parts by weight of a Ketchen black powder wereadded to and mixed with 100 parts by weight of the alloy powder thusprepared. Further, a paste was prepared by adding 1 parts by weight ofstyrene butadiene rubber (SBR), 0.2 parts by weight of sodiumpolyacrylate, 0.2 parts by weight of carboxymethyl cellulose, and 50parts by weight of water to 100 parts by weight of the mixed powder,followed by stirring the mixture. Still further, a punched thin ironplate having a nickel plating applied to the surface was coated with thepaste thus obtained, followed by drying the paste so as to obtain acoated plate. The thickness of the coated plate thus obtained wasadjusted by applying a roll press to the coated plate, followed bycutting the coated plate into pieces each having a desired size so as toprepare a negative electrode containing 3.5 g of the hydrogen absorbingalloy.

On the other hand, prepared as a separator was a polyolefin basedunwoven fabric having acrylic acid introduced therein by a graftcopolymerization.

An electrode group was prepared by spirally winding a laminate structurecomprising the negative electrode prepared as described above, a pastetype nickel positive electrode having a nominal capacity of 700 mAh,which had been prepared by the known method, and the separator describedabove, which was interposed between the positive electrode and thenegative electrode.

The electrode group thus prepared was put in a cylindrical case togetherwith 1.5 ml of an alkaline electrolyte containing 7 mol of KOH and 1 molof LiOH, followed by sealing the case so as to assemble a cylindricalnickel hydrogen secondary battery of size AAA having a nominal capacityof 700 mAh.

Each of the secondary batteries prepared in Examples 41 to 56 andComparative Examples 11 to 15 was left to stand at room temperature for24 hours. Then, a charge-discharge cycle, in which the secondary batterywas charged under a current of 70 mA for 15 hours, followed bydischarging the secondary battery under a current of 70 mA until thebattery voltage was lowered to 0.7V, was carried out four times.Further, the charge-discharge cycle was repeated under an environment of45° C. so as to measure the number of cycles reached at the time whenthe discharge capacity of the secondary battery was lowered to 80% ofthe discharge capacity of the first cycle. Table 10 shows the number ofcycles and the discharge capacity of the first cycle. Incidentally, thecharging process of the charge-discharge cycle was performed byemploying a −ΔV method in which the charging current was set at 700 mAand the charging was finished at the time when the voltage was loweredby 10 mV from the maximum voltage in the charging process. On the otherhand, the discharge process was performed under a current of 1,400 mAuntil the battery voltage was lowered to 1.0V.

Also, characteristics (A) to (E) given below were measured in respect ofthe hydrogen absorbing alloy used in the secondary battery for each ofExamples 41 to 56 and Comparative Examples 11 to 15:

(A) A rechargeable hydrogen storage capacity (which is a term for ahydrogen absorbing alloy specified in JIS H7003) was obtained as ahydrogen absorption characteristic for each of the hydrogen absorbingalloys by measuring the iso-thermal line of pressure-composition at 50°C. and under a hydrogen pressure lower than 10 atm. by the Sievert'smethod (JIS H7201). Table 10 shows the results.

(B) The intensity ratio (I₁/I₂) of each of the hydrogen absorbing alloyswas calculated from the X-ray diffraction pattern by using a Cu—Kα rayas the X-ray source. Table 10 shows the results. Incidentally, I₂ usedfor calculating the intensity ratio denotes an intensity of a peakhaving a highest intensity in the X-ray diffraction pattern. On theother hand, I₁ denotes an intensity of a peak having a highest intensityobserved at the value of 2θ falling within a range of from 8° to 13°.

(C) The crystal structure of each of the hydrogen absorbing alloys wasobserved from the X-ray diffraction pattern obtained in item (B) givenabove so as to determine the crystal structure of the principal phase.Table 10 shows the results.

In the hydrogen absorbing alloy for Example 53, the peak having thehighest intensity appeared at the value of 2θ falling within a range of42.1°±1° in the X-ray diffraction pattern, and a peak appeared at thevalue of 2θ falling within a range of from 31° to 34° was found as beingsplit into three. The intensity of the three-split peak was lower thanthat of peak P1 in diffraction pattern (a) of the normal structurereferred to previously. On the other hand, in the hydrogen absorbingalloy for Example 54, the peak having the highest intensity appeared atthe value of 2θ falling within a range of 42.1°±1° in the X-raydiffraction pattern, and a peak having an intensity lower than that ofpeak P1 referred previously appeared at the value of 2θ falling within arange of from 31° to 34°. Incidentally, the intensity ratio calculatedby formula (I) referred to previously on the basis of the diffractionpattern of the hydrogen absorbing alloy for Example 54 was found to be23%.

Further, the electron diffraction analysis pattern for the hydrogenabsorbing alloy for each of Examples 53 and 54 was photographed by atransmission electron microscope (TEM). It has been found that therewere four supper lattice reflection spots between the primitive latticereflection points (00L) and the origin (000) in the hydrogen absorbingalloy for each of Examples 53 and 54. It has also been found that thesupper lattice reflection spots were present in four points at which thedistance |G_(00L)| between the primitive lattice reflection spot (00L)and the origin (000) was equally divided into five sections.

Also, the peak having the highest intensity appeared at the value of 2θfalling within a range of 42.1°±1° in the X-ray diffraction pattern ofthe hydrogen absorbing alloy for Example 55. Further, a peak appeared atthe value of 2θ falling within a range of from 31° to 34° in the X-raydiffraction pattern was found as being split into four. The intensity ofthe four-split peak was found to be lower than that of the peakappearing in the diffraction pattern of the normal structure referred topreviously. On the other hand, the peak having the highest intensityappeared at the value of 2θ falling within a range of 42.1°±1° in theX-ray diffraction pattern of the hydrogen absorbing alloy for Example56. Also, a peak having an intensity lower than that of peak of thenormal structure referred to previously appeared at the value of 2θfalling within a range of from 31° to 34°. Incidentally, the intensityratio calculated by formula (I) referred to previously on the basis ofthe diffraction pattern of the hydrogen absorbing alloy for Example 56was found to be 19%.

(D) A secondary electron image and a reflected electron image of each ofthe hydrogen absorbing alloys was photographed by using a scanningelectron microscope (SEM) so as to detect a phase differing from theprincipal phase. The composition of the phase differing from theprincipal phase was analyzed by an energy dispersive X-ray spectroscopy(EDX) of the scanning electron microscope. It has been found from theresult of the composition analysis and the X-ray diffraction patternobtained in item (B) referred to above that the phase differing from theprincipal phase was a phase having a MgCu₂ type crystal structure.

(E) The content of the MgCu₂ type phase was measured as follows for eachof the hydrogen absorbing alloys so as to obtain the results as shown inTable 10.

Optional five view fields of the hydrogen absorbing alloy for each ofExamples 41 to 48, 53 to 56 and Comparative Examples 12, 14, 15 werephotographed by a scanning electron microscope. The area ratio of thetarget phase based on the alloy area within the view field was obtainedfor each of the micrographs. The average value of the area ratios thusobtained was calculated and given in Table 10 as the volume ratio of thetarget phase in the hydrogen absorbing alloy.

On the other hand, optional five view fields of the hydrogen absorbingalloy for each of Examples 49 to 52 and Comparative Examples 11, 13 werephotographed by a transmission electron microscope. The area ratio ofthe target phase based on the alloy area within the view field wasobtained for each of the micrographs. The average value of the arearatios thus obtained was calculated and given in Table 10 as the volumeratio of the target phase in the hydrogen absorbing alloy.

TABLE 9 Heat treatment conditions Temperature Time Composition (° C.)(h) Example 41 Lm(7)_(0.77)Mg_(0.23)Ni_(3.3)Al_(0.12) 945 10  Example 42La_(0.6)Pr_(0.14)Mg_(0.26)Ni_(3.29)Mn_(0.02)Si_(0.01)Al_(0.07) 950 8Example 43 La_(0.53)Nd_(0.2)Mg_(0.27)Ni_(3.1)Co_(0.12)Fe_(0.01)Al_(0.1)955 6 Example 44 Lm(5)_(0.77)Mg_(0.23)Ni_(3.3)Al_(0.11)Mo_(0.003) 920 5Example 45Lm(8)_(0.8)Mg_(0.2)Ni_(3.34)Co_(0.05)Mn_(0.02)Al_(0.1)Ta_(0.005) 985 7Example 46La_(0.77)Ce_(0.03)Mg_(0.2)Ni_(3.45)Zn_(0.02)Al_(0.12)Li_(0.005) 920 6Example 47Lm(5)_(0.74)Y_(0.07)Mg_(0.19)Ni_(3.24)Co_(0.08)Sn_(0.01)Al_(0.08) 915 9Example 48 Lm(6)_(0.7)Mg_(0.3)Ni_(3.23)Cu_(0.02)Al_(0.09) 935 7 Example49 Lm(7)_(0.76)Mg_(0.24)Ni_(3.3)Co_(0.02)W_(0.002)B_(0.01)Al_(0.11) 9502 Example 50Lm(5)_(0.68)Mm(3)_(0.11)Mg_(0.21)Ni_(3.14)Co_(0.2)Al_(0.12)Ga_(0.03) 9351 Example 51Lm(5)_(0.76)Mg_(0.24)Ni_(3.3)Co_(0.06)Mn_(0.03)V_(0.002)Al_(0.07) 915 3Example 52 Lm(8)_(0.76)Ca_(0.01)Mg_(0.23)Ni_(3.3)Cr_(0.002)Al_(0.11) 9752 Example 53 Lm(6)_(0.765)Zr_(0.005)Mg_(0.23)Ni_(3.31)P_(0.002)Al_(0.1)935 4 Example 54Lm(7)_(0.775)Ti_(0.005)Mg_(0.22)Ni_(3.36)Al_(0.15)S_(0.003) 940 3Example 55 Lm(7)_(0.77)Mg_(0.23)Ni_(3.16)Co_(0.063)Al_(0.12) 940 5Example 56Lm(5)_(0.73)Mm(3)_(0.05)Mg_(0.22)Ni_(3.08)Co_(0.1)Mn_(0.01)Sn_(0.02)Al_(0.07)950 4 ComparativeMm(3)_(0.65)Mg_(0.35)Ni_(2.27)Co_(0.3)Mn_(0.3)Fe_(0.02)Al_(0.14) 900 4Example 11 ComparativeMm(3)_(0.975)Mg_(0.025)Ni_(2.925)Mn_(0.35)Cu_(0.1)Nb_(0.075) 900 12 Example 12 Comparative Lm(5)_(0.917)Mg_(0.083)Ni_(4.75) — — Example 13Comparative La_(0.33)Mg_(0.67)Ni₃ 1000  4 Example 14 ComparativeLm(5)_(0.5)Mg_(0.5)Ni_(2.2) 800 5 Example 15 Note: The alloys forExamples 49 to 52 and Comparative Example 11 were prepared by a rapidsolidification process, and the alloy for Comparative Example 13 wasprepared by a mechanical alloying method.

TABLE 10 Content Rechargeable of AB₂ hydrogen type storage Intensityphase Capacity Cycle capacity Crystal structure of ratio (% by (mAh)life (H/M) principal phase (I₁/I₂) volume) Example 41 610 320 1.07Ce₂Ni₇ type 0.005 0.1 Example 42 605 320 1.03 Ce₂Ni₇ type + PuNi₃ type0.008 0.2 Example 43 610 295 1.06 Ce₂Ni₇ type + CeNi₃ type 0 0.8 Example44 600 290 1.05 Ce₂Ni₇ type + PuNi₃ type 0.002 0.5 Example 45 605 2951.08 Ce₂Ni₇ type 0.01 0.8 Example 46 595 285 0.94 Ce₂Ni₇ type + PuNi₃type 0.004 1.9 Example 47 590 280 0.92 Ce₂Ni₇ type 0.009 2.6 Example 48585 285 1.08 Ce₂Ni₇ type + PuNi₃ type 0 2.4 Example 49 595 270 0.94Ce₂Ni₇ type 0.007 2.9 Example 50 600 265 0.96 Ce₂Ni₇ type + Gd₂Co₇ type0.011 4.6 Example 51 610 310 0.99 Ce₂Ni₇ type 0.008 0.4 Example 52 615230 0.97 Ce₂Ni₇ type 0.006 2.2 Example 53 610 295 0.89 Similar to Ce₂Ni₇type 0.01 1.1 Example 54 615 275 0.87 Similar to Ce₂Ni₇ type 0 3.6Example 55 610 250 0.86 Similar to PuNi₃ type 0.003 4.1 Example 56 605240 0.85 Similar to PuNi₃ type 0.007 5.1 Comparative 550 75 0.7 CaCu₅type 0.08 14 Example 11 Comparative 560 25 0.58 Ce₂Ni₇ type 0.002 17Example 12 Comparative 460 10 0.44 CaCu₅ type 0 0 Example 13 Comparative540 10 0.25 PuNi₃ type 0.23 19 Example 14 Comparative 420 15 0.34 MgCu₂type 0.17 78 Example 15

To reiterate, the hydrogen absorbing alloy for each of Examples 41 to 56had a composition represented by formula (3) given previously and hadless than 0.15 of the intensity ratio calculated by formula (2) givenpreviously. Also, the amount of the AB₂ type phase such as the MgCu₂type phase contained in the hydrogen absorbing alloy was not larger than10% by volume. As apparent from Tables 9 and 10, the rechargeablehydrogen storage capacity of the hydrogen absorbing alloy for each ofExamples 41 to 56 was larger than that of the hydrogen absorbing alloyfor each of Comparative Examples 11 to 15. Incidentally, the hydrogenabsorbing alloy for Comparative Example 11 had a composition equal tothat of the hydrogen absorbing alloy disclosed in U.S. Pat. No.5,840,166. And the hydrogen absorbing alloy for Comparative Example 11contained the AB₂ type phase in an amount exceeding 10% by volume. Also,the hydrogen absorbing alloy for Comparative Example 12 had acomposition equal to that of the hydrogen absorbing alloy disclosed inJapanese Patent Disclosure No. 11-29832. And the hydrogen absorbingalloy for Comparative Example 12 contained the AB₂ type phase in anamount exceeding 10% by volume. On the other hand, the hydrogenabsorbing alloy for Comparative Example 13 had a composition equal tothat of the hydrogen absorbing alloy disclosed in Japanese PatentDisclosure No. 10-1731.

It is also seen that the secondary battery for each of Examples 41 to 56was found to be superior to the secondary battery for each ofComparative Examples 11 to 15 in each of the discharge capacity and thecharge-discharge cycle life.

EXAMPLES 57 TO 72 AND COMPARATIVE EXAMPLES 16 TO 20

Hydrogen absorbing alloys were prepared by the high frequency inductionmelting method, the rapid solidification process or the mechanicalalloying method as described in the following.

(High Frequency Induction Melting Method)

Each of the elements constituting the composition shown in Table 11 wasweighed, followed by melting the composition in a high frequencyinduction furnace under an argon gas atmosphere so as to obtain an alloyingot. Then, a heat treatment was applied to the alloy ingot thusobtained under an argon gas atmosphere and under the conditions shown inTable 11 so as to obtain hydrogen absorbing alloys for Examples 57 to 65and 69 to 72 and Comparative Examples 17, 19, 20.

(Rapid Solidification Process)

Each of the elements constituting the composition shown in Table 11 wasweighed, followed by melting the composition in a high frequencyinduction furnace under an argon gas atmosphere so as to obtain an alloyingot. The alloy ingot thus obtained was melted, followed by drippingthe resultant melt onto the surface of a single roll made of copper androtating at a peripheral velocity of 7 m/sec under an argon gasatmosphere so as to rapidly cool the melt and, thus, to obtain aflake-like hydrogen absorbing alloy. Further, a heat treatment wasapplied to the resultant hydrogen absorbing alloy flakes under an argongas atmosphere and under the conditions shown in Table 11 so as toobtain hydrogen absorbing alloys for Examples 66 to 68 and ComparativeExample 16.

(Mechanical Alloying Method)

A raw material metal mixture prepared by mixing Lm(6) and Ni at anatomic ratio Lm(6):Ni of 1:3 was melted in an arc furnace, followed bycooling the melt so as to obtain an alloy (a) consisting of a Lm(6)Ni₃phase. Also, another raw material metal mixture prepared by mixing Lm(6)and Ni at an atomic ratio Lm(6):Ni of 1:4 was melted in an arc furnace,followed by cooling the melt so as to obtain an alloy (b) consisting ofa Lm(6)Ni₄ phase. The alloy (a) in an amount of 100 g and the alloy (b)in an amount of 100 g were put in a planetary ball mill having steelballs put therein so as to be subjected to a mechanical alloying for 10hours at room temperature under an argon gas atmosphere, therebyobtaining an alloy (c) consisting of a Lm(6)Ni₃ phase, a Lm(6)Ni₄ phaseand a Lm(6)₅Ni₁₉ phase.

On the other hand, a raw material metal mixture prepared by mixingLm(6), Mg and Ni at an atomic ratio Lm(6):Mg:Ni of 0.9:0.1:5 was meltedin an arc furnace, followed by cooling the melt so as to obtain an alloy(d) consisting of a Lm(6)_(0.9)Mg_(0.1)Ni₅ phase. The alloy (d) in anamount of 30 g and the alloy (c) in an amount of 300 g were put in aplanetary ball mill having steel balls put therein so as to be subjectedto a mechanical alloying for 10 hours at room temperature under an argongas atmosphere, thereby obtaining an alloy for Comparative Example 18consisting of a Lm(6)Ni₃ phase, a Lm(6)Ni₄ phase, a Lm(6)₅Ni₁₉ phase anda Lm(6)_(0.9)Mg_(0.1)Ni₅ phase.

A cylindrical nickel hydrogen secondary battery was assembled as inExample 41 by using each of the hydrogen absorbing alloys thus prepared.

The secondary battery prepared in each of Examples 57 to 72 andComparative Examples 16 to 20 was left to stand under room temperaturefor 24 hours. Then, each of the discharge capacity and thecharge-discharge cycle life of the secondary battery were measured as inExample 41 so as to obtain the results shown in Table 12.

Also, characteristics (A) to (F) given below were measured in respect ofthe hydrogen absorbing alloy used in the secondary battery for each ofExamples 57 to 72 and Comparative Examples 16 to 20:

(A) A rechargeable hydrogen storage capacity was obtained for each ofthe hydrogen absorbing alloys as in Example 41. Table 12 shows theresults.

(B) The intensity ratio (I₁/I₂) of each of the hydrogen absorbing alloyswas calculated from the X-ray diffraction pattern by using a Cu—Kα rayas the X-ray source. Table 12 shows the results.

(C) The crystal structure of each of the hydrogen absorbing alloys wasobserved from the X-ray diffraction pattern obtained in item (B) aboveso as to determine the crystal structure of the principal phase. Table12 shows the results.

In the hydrogen absorbing alloy for Example 69, the peak having thehighest intensity appeared at the value of 2θ falling within a range of42.1°±1° in the X-ray diffraction pattern, and a peak appeared at thevalue of 2θ falling within a range of from 31° to 34° was found as beingsplit into three. The intensity of the three-split peak was lower thanthat of peak P1 in diffraction pattern (a) of the normal structurereferred to previously. On the other hand, in the hydrogen absorbingalloy for Example 70, the peak having the highest intensity appeared atthe value of 2θ falling within a range of 42.1°±1° in the X-raydiffraction pattern, and a peak having an intensity lower than that ofpeak P1 referred previously appeared at the value of 2θ falling within arange of from 31° to 34°. Incidentally, the intensity ratio calculatedby formula (I) referred to previously on the basis of the diffractionpattern of the hydrogen absorbing alloy for Example 70 was found to be26%.

Further, the electron diffraction analysis pattern for the hydrogenabsorbing alloy for each of Examples 69 and 70 was photographed by atransmission electron microscope (TEM). It has been found that therewere four supper lattice reflection spots between the primitive latticereflection points (00L) and the origin (000) in the hydrogen absorbingalloy for each of Examples 69 and 70. It has also been found that thesupper lattice reflection spots were present in four points at which thedistance |G_(00L)| between the primitive lattice reflection spot (00L)and the origin (000) was equally divided into five sections.

Also, the peak having the highest intensity appeared at the value of 2θfalling within a range of 42.1°±1° in the X-ray diffraction pattern ofthe hydrogen absorbing alloy for Example 71. Further, a peak appeared atthe value of 2θ falling within a range of from 31° to 34° was found asbeing split into two. The intensity of the two-split peak was found tobe lower than that of the peak appearing in the diffraction pattern ofthe normal structure referred to previously. On the other hand, the peakhaving the highest intensity appeared at the value of 2θ falling withina range of 42.1°±1° in the X-ray diffraction pattern of the hydrogenabsorbing alloy for Example 72. Also, a peak having an intensity lowerthan that of peak of the normal structure referred to previouslyappeared at the value of 2θ falling within a range of from 31° to 34°.Incidentally, the intensity ratio calculated by formula (I) referred topreviously on the basis of the diffraction pattern of the hydrogenabsorbing alloy for Example 72 was found to be 21%.

(D) The volume ratio of the parallel growth region in the crystal grainwas measured as in Example 17 for each of the hydrogen absorbing alloys.Table 12 shows the results.

(E) A ratio of the number of crystal grains, in which the volume ratioof the parallel growth region was not higher than 40%, to the totalnumber of crystal grains was calculated as in Example 17 and given inTable 12, said ratio being hereinafter referred to as a “crystal grainratio”.

(F) The composition of the parallel growth region excluding theprincipal phase of each of the hydrogen absorbing alloys was analyzed byan energy dispersive X-ray spectroscopy (EDX) of the scanning electronmicroscope. The crystal structure of the parallel growth region wasspecified from the result of the composition analysis and the X-raydiffraction pattern obtained in item (B) described previously. Theresults are shown in Table 12.

TABLE 11 Heat treatment conditions Temperature Time Composition (° C.)(h) Example 57 Lm(5)_(0.78)Mg_(0.22)Ni_(3.34)Al_(0.12) 915 6 Example 58Lm(5)_(0.62)Pr_(0.14)Mg_(0.24)Ni_(3.29)Mn_(0.02)Fe_(0.01)Al_(0.09) 950 5Example 59 La_(0.63)Nd_(0.12)Mg_(0.25)Ni_(3.1)Co_(0.1)Si_(0.01)Al_(0.1)970 11  Example 60 Lm(7)_(0.77)Mg_(0.23)Ni_(3.35)Al_(0.1)Ta_(0.003) 9456 Example 61Lm(6)_(0.8)Mg_(0.2)Ni_(3.4)Co_(0.05)Mn_(0.02)Al_(0.1)Mo_(0.002) 930 7Example 62La_(0.72)Ce_(0.05)Mg_(0.23)Ni_(3.35)Zn_(0.03)Al_(0.12)Li_(0.003) 915 6Example 63 Lm(6)_(0.74)Y_(0.07)Mg_(0.19)Ni_(3.45)Co_(0.1)Sn_(0.01) 930 5Example 64 Lm(5)_(0.74)Mg_(0.26)Ni_(3.25)Cu_(0.02)Al_(0.1) 915 4 Example65 Lm(7)_(0.78)Mg_(0.22)Ni_(3.3)Co_(0.02)W_(0.002)B_(0.003)Al_(0.1) 9554 Example 66Lm(5)_(0.74)Mm(3)_(0.05)Mg_(0.21)Ni_(3.32)Al_(0.12)Ga_(0.03) 915 3Example 67Lm(6)_(0.76)Mg_(0.24)Ni_(3.3)Co_(0.06)Mn_(0.03)V_(0.002)Al_(0.1) 935 4Example 68 Lm(8)_(0.76)Ca_(0.01)Mg_(0.23)Ni_(3.3)Cr_(0.003)Al_(0.12) 9752 Example 69 Lm(5)_(0.768)Zr_(0.002)Mg_(0.23)Ni_(3.31)P_(0.002)Al_(0.1)925 8 Example 70Lm(7)_(0.787)Ti_(0.003)Mg_(0.21)Ni_(3.36)Al_(0.13)S_(0.003) 950 5Example 71 Lm(6)_(0.77)Mg_(0.23)Ni_(3.18)Co_(0.09)Al_(0.12) 945 7Example 72Lm(5)_(0.76)Mm(3)_(0.02)Mg_(0.22)Ni_(3.02)Co_(0.2)Mn_(0.01)Sn_(0.02)Al_(0.1)950 9 ComparativeMm(3)_(0.72)Mg_(0.28)Ni_(2.66)Co_(0.4)Mn_(0.4)Fe_(0.02)Al_(0.15) 900 4Example 16 ComparativeMm(3)_(0.975)Mg_(0.025)Ni₃Mn_(0.35)Cu_(0.1)Ga_(0.05) 900 12  Example 17Comparative Lm(6)_(0.917)Mg_(0.083)Ni_(4.75) — — Example 18 ComparativeLm(6)_(0.33)Mg_(0.67)Ni₃ 1000  5 Example 19 ComparativeLm(5)_(0.55)Mg_(0.45)Ni_(2.3) 800 7 Example 20 Note: The alloys forExamples 66 to 68 and Comparative Example 16 were prepared by a rapidsolidification process, and the alloy for Comparative Example 18 wasprepared by a mechanical alloying method.

TABLE 12 Rechargeable Parallel hydrogen Crystal growth Crystal Crystalstorage Intensity structure of content of grain structure of CapacityCycle capacity ratio principal crystal grain ratio parallel (mAh) life(H/M) (I₁/I₂) phase (% by volume) (%) growth Example 57 1355 325 1.060.003 Ce₂Ni₇ type 9 93 PuNi₃ type + A₅B₁₉ type Example 58 1350 315 1.040.002 Ce₂Ni₇ type 6 90 PuNi₃ type Example 59 1335 290 1.05 0 Ce₂Ni₇type + CeNi₃ 16 86 CeNi₃ type + A₅B₁₉ type type Example 60 1360 280 1.060.002 Ce₂Ni₇ type + PuNi₃ 14 87 PuNi₃ type + A₅B₁₉ type type Example 611365 295 1.03 0 Ce₂Ni₇ type 7 91 PuNi₃ type Example 62 1350 285 1.040.005 Ce₂Ni₇ type + PuNi₃ 17 84 A₅B₁₉ type type Example 63 1335 280 1.020.002 Ce₂Ni₇ type 19 83 CeNi₃ type + A₅B₁₉ type Example 64 1360 285 0.990.003 Ce₂Ni₇ type 21 85 A₅B₁₉ type Example 65 1355 270 1.01 0.004 Ce₂Ni₇type 22 75 PuNi₃ type + A₅B₁₉ type Example 66 1360 265 0.95 0.009 Ce₂Ni₇type + Gd₂Co₇ 31 70 CeNi₃ type + A₅B₁₉ type type Example 67 1335 3200.98 0.012 Ce₂Ni₇ type 6 90 PuNi₃ type + A₅B₁₉ type Example 68 1360 2250.97 0.01 Ce₂Ni₇ type 9 87 PuNi₃ type + A₅B₁₉ type Example 69 1365 2950.89 0.011 Similar to 11 76 CeNi₃ type + A₅B₁₉ Ce₂Ni₇ type type Example70 1365 275 0.87 0.007 Similar to 17 80 PuNi₃ type + A₅B₁₉ Ce₂Ni₇ typetype Example 71 1350 250 0.85 0.005 Similar to 32 66 Ce₂Ni₇ type + A₅B₁₉PuNi₃ type type Example 72 1335 220 0.86 0.004 Similar to 39 61 Ce₂Ni₇type + A₅B₁₉ PuNi₃ type type Comparative 1240 70 0.66 0.03 CaCu₅ type 6053 Ce₂Ni₇ type Example 16 Comparative 1000 25 0.55 0.01 Ce₂Ni₇ type 5265 CaCu₅ type + PuNi₃ Example 17 type Comparative 720 10 0.5 0 CaCu₅type 5 85 Ce₂Ni₇ type + A₅B₁₉ Example 18 type Comparative 680 15 0.220.28 PuNi₃ type 15 78 Ce₂Ni₇ type + A₅B₁₉ Example 19 type Comparative740 15 0.39 0.31 MgCu₂ type 28 55 PuNi₃ type Example 20

To reiterate, the hydrogen absorbing alloy for each of Examples 57 to 72had a composition represented by formula (3) given previously and hadless than 0.15 of the intensity ratio calculated by formula (2) givenpreviously. Also, the parallel growth region that has a crystalstructure differing from the crystal structure of the principal phaseprecipitates in at least one crystal grain of the principal phase. Asapparent from Tables 11 and 12, the rechargeable hydrogen storagecapacity of the hydrogen absorbing alloy for each of Examples 57 to 72was larger than that of the hydrogen absorbing alloy for each ofComparative Examples 16 to 20. Incidentally, the hydrogen absorbingalloy for Comparative Example 16 had a composition equal to that of thehydrogen absorbing alloy disclosed in U.S. Pat. No. 5,840,166 andcontained the CaCu₅ type phase as the principal phase. Also, thehydrogen absorbing alloy for Comparative Example 17 had a compositionequal to that of the hydrogen absorbing alloy disclosed in JapanesePatent Disclosure No. 11-29832 and contained the Ce₂Ni₇ type phase asthe principal phase. On the other hand, the hydrogen absorbing alloy forComparative Example 18 had a composition equal to that of the hydrogenabsorbing alloy disclosed in Japanese Patent Disclosure No. 10-1731.

It is also seen that the secondary battery for each of Examples 57 to 72was found to be superior to the secondary battery for each ofComparative Examples 16 to 20 in each of the discharge capacity and thecharge-discharge cycle life.

EXAMPLES 73 TO 80

Hydrogen absorbing alloys were prepared by the high frequency inductionmelting method as described in the following.

(High Frequency Induction Melting Method)

Each of the elements constituting the composition shown in Table 13 wasweighed, followed by melting the composition in a high frequencyinduction furnace under an argon gas atmosphere so as to obtain an alloyingot. Then, a heat treatment was applied to the alloy ingot thusobtained under an argon gas atmosphere and under the conditions shown inTable 13 so as to obtain hydrogen absorbing alloys for Examples 73 to80.

A cylindrical nickel hydrogen secondary battery was assembled as inExample 41 by using each of the hydrogen absorbing alloys thus prepared.

The secondary battery prepared in each of Examples 73 to 80 was left tostand under room temperature for 24 hours. Then, each of the dischargecapacity and the charge-discharge cycle life of the secondary batterywere measured as in Example 41 so as to obtain the results shown inTable 13.

The rechargeable hydrogen storage capacity, the intensity ratio (I₁/I₂),the crystal structure of the principal phase, the content of the AB₂type phase, the content of the parallel growth region in the crystalgrain, the crystal grain ratio, and the crystal structure of theparallel growth region were measured as in Examples 41 and 57 in respectof the hydrogen absorbing alloy used in the secondary battery for eachof Examples 73 to 80. Tables 13 and 14 show the results.

TABLE 13 Rechargeable Heat treatment hydrogen conditions storageTemperature Time Capacity Cycle capacity Composition (° C.) (h) (mAh)life (H/M) Example 73 Lm(5)_(0.77)Mg_(0.23)Ni_(3.3)Al_(0.12) 915 8 1350335 1.06 Example 74 Lm(6)_(0.77)Mg_(0.23)Ni_(3.24)Co_(0.03)Al_(0.11) 9356 1340 330 1.04 Example 75Lm(7)_(0.76)Mg_(0.24)Ni_(3.24)Mn_(0.02)Al_(0.13) 955 7 1345 320 1.03Example 76 Lm(7)_(0.79)Mg_(0.21)Ni_(3.23)Mn_(0.08)Al_(0.12) 975 5 1360330 1.05 Example 77Lm(8)_(0.78)Mg_(0.22)Ni_(3.15)Co_(0.1)Mn_(0.03)Al_(0.13) 970 8 1365 2951.03 Example 78 Lm(7)_(0.77)Mg_(0.23)Ni_(3.24)Cu_(0.03)Al_(0.12) 950 101350 325 1.06 Example 79 Lm(6)_(0.77)Mg_(0.23)Ni_(3.3)Al_(0.12) 935 51365 330 1.03 Example 80 Lm(7)_(0.77)Mg_(0.23)Ni_(3.32)Al_(0.09) 945 71360 340 1.05

TABLE 14 Parallel Content of Crystal growth Crystal Crystal IntensityAB₂ type structure of content of grain structure ratio phase (%principal crystal grain ratio of parallel (I₁/I₂) by volume) phase (% byvolume) (%) growth Example 73 0.003 0.1 Ce₂Ni₇ type 6 93 PuNi₃ type +A₅B₁₉ type Example 74 0 0.5 Ce₂Ni₇ type + PuNi₃ 7 95 CeNi₃ type + A₅B₁₉type type Example 75 0.005 1.1 Ce₂Ni₇ type + CeNi₃ 8 90 PuNi₃ type +A₅B₁₉ type type Example 76 0.002 1.5 Ce₂Ni₇ type + PuNi₃ 9 92 PuNi₃type + A₅B₁₉ type type Example 77 0 0.8 Ce₂Ni₇ type 6 92 PuNi₃ type +A₅B₁₉ type Example 78 0.004 0.4 Ce₂Ni₇ type 7 93 CeNi₃ type + A₅B₁₉ typeExample 79 0.009 0.7 Ce₂Ni₇ type 7 91 PuNi₃ type + A₅B₁₉ type Example 800.013 0.9 Ce₂Ni₇ type + PuNi₃ 8 95 CeNi₃ type + A₅B₁₉ type type

As apparent from Tables 13 and 14, the secondary battery for each ofExamples 73 to 80 has a large capacity and a long charge-discharge cyclelife.

Table 15 shows the compositions of mish metals Lm and Mm referred toherein later in Tables 16, 18 and 20:

TABLE 15 (Lm, Mm shown in Tables 16, 18, 20 . . . unit: wt %) La Ce PrNd Sm Lm(9) 97 0.1 0.8 2.1 — Lm(10) 84 1.1 2.1 12.8 — Lm(11) 73 2.3 5.818.9 — Lm(12) 52 2.9 14.1 31 — Mm(4) 42 49 2.1 6.8 0.1

EXAMPLES 81 TO 96 AND COMPARATIVE EXAMPLES 21 TO 25

Hydrogen absorbing alloys were prepared by the high frequency inductionmelting method, the rapid solidification process or the mechanicalalloying method as described in the following.

(High Frequency Induction Melting Method)

Each of the elements constituting the composition shown in Table 16 wasweighed, followed by melting the composition in a high frequencyinduction furnace under an argon gas atmosphere so as to obtain an alloyingot. Then, a heat treatment was applied to the alloy ingot thusobtained under an argon gas atmosphere and under the conditions shown inTable 16 so as to obtain hydrogen absorbing alloys for Examples 81 to88, 93 to 96 and Comparative Examples 22, 24, 25.

(Rapid Solidification Process)

Each of the elements constituting the composition shown in Table 16 wasweighed, followed by melting the composition in a high frequencyinduction furnace under an argon gas atmosphere so as to obtain an alloyingot. The alloy ingot thus obtained was melted, followed by drippingthe resultant melt onto the surface of a single roll made of copper androtating at a peripheral velocity of 12 m/sec under an argon gasatmosphere so as to rapidly cool the melt and, thus, to obtain aflake-like hydrogen absorbing alloy. Further, a heat treatment wasapplied to the resultant hydrogen absorbing alloy flakes under an argongas atmosphere and under the conditions shown in Table 16 so as toobtain hydrogen absorbing alloys for Examples 89 to 92 and ComparativeExample 21.

(Mechanical Alloying Method)

A raw material metal mixture prepared by mixing Lm(10) and Ni at anatomic ratio Lm(10):Ni of 1:3 was melted in an arc furnace, followed bycooling the melt so as to obtain an alloy (a) consisting of a Lm(10)Ni₃phase. Also, another raw material metal mixture prepared by mixingLm(10) and Ni at an atomic ratio Lm(10):Ni of 1:4 was melted in an arcfurnace, followed by cooling the melt so as to obtain an alloy (b)consisting of a Lm(10)Ni₄ phase. The alloy (a) in an amount of 100 g andthe alloy (b) in an amount of 100 g were put in a planetary ball millhaving steel balls put therein so as to be subjected to a mechanicalalloying for 10 hours at room temperature under an argon gas atmosphere,thereby obtaining an alloy (c) consisting of a Lm(10)Ni₃ phase, aLm(10)Ni₄ phase and a Lm(10)₅Ni₁₉ phase.

On the other hand, a raw material metal mixture prepared by mixingLm(10), Mg and Ni at an atomic ratio Lm(10):Mg:Ni of 0.9:0.1:5 wasmelted in an arc furnace, followed by cooling the melt so as to obtainan alloy (d) consisting of a Lm(10)_(0.9)Mg_(0.1)Ni₅ phase. The alloy(d) in an amount of 30 g and the alloy (c) in an amount of 300 g wereput in a planetary ball mill having steel balls put therein so as to besubjected to a mechanical alloying for 10 hours at room temperatureunder an argon gas atmosphere, thereby obtaining an alloy forComparative Example 23 consisting of a Lm(10)Ni₃ phase, a Lm(10)Ni₄phase, a Lm(10)₅Ni₁₉ phase and a Lm(10)_(0.9)Mg_(0.1)Ni₅ phase.

Each of the hydrogen absorbing alloys thus prepared was pulverized underan inert gas atmosphere so as to permit the pulverized powder to have anaverage particle diameter of 60 μm. Then, 0.5 parts by weight of anickel powder available on the market, which had been prepared by thecarbonyl method, and 0.5 parts by weight of a Ketchen black powder wereadded to and mixed with 100 parts by weight of the alloy powder thusprepared. Further, a paste was prepared by adding 1 parts by weight ofstyrene butadiene rubber (SBR), 0.2 parts by weight of sodiumpolyacrylate, 0.2 parts by weight of carboxymethyl cellulose, and 50parts by weight of water to 100 parts by weight of the mixed powder,followed by stirring the mixture. Still further, a punched thin ironplate having a nickel plating applied to the surface was coated with thepaste thus obtained, followed by drying the paste so as to obtain acoated plate. The thickness of the coated plate thus obtained wasadjusted by applying a roll press to the coated plate, followed bycutting the coated plate into pieces each having a desired size so as toprepare a negative electrode.

On the other hand, prepared as a separator was a polyolefin basedunwoven fabric having acrylic acid introduced therein by a graftcopolymerization.

An electrode group was prepared by alternately laminating the negativeelectrode prepared as described above and a paste type nickel positiveelectrode prepared by the known method, with the separator describedabove interposed between the positive electrode and the negativeelectrode. The electrode group thus prepared contained 4.2 g of thehydrogen absorbing alloy. Also, the nominal capacity of the paste typenickel positive electrode included in the electrode group was 830 mAh.

The electrode group thus prepared was put in a cylindrical case togetherwith 1.3 ml of an alkaline electrolyte containing 7 mol of KOH, 0.5 molof NaOH, and 0.5 mol of LiOH, followed by sealing the case so as toassemble a rectangular nickel hydrogen secondary battery of size F6having a nominal capacity of 830 mAh.

Each of the secondary batteries prepared in Examples 81 to 96 andComparative Examples 21 to 25 was left to stand at room temperature for72 hours. Then, a charge-discharge cycle, in which the secondary batterywas charged under a current of 83 mA for 15 hours, followed bydischarging the secondary battery under a current of 166 mA until thebattery voltage was lowered to 0.7V, was carried out two times. Further,the charge-discharge cycle was repeated under an environment of 45° C.so as to measure the number of cycles reached at the time when thedischarge capacity of the secondary battery was lowered to 80% of thedischarge capacity of the first cycle. Table 17 shows the number ofcycles and the discharge capacity of the first cycle. Incidentally, thecharging process of the charge-discharge cycle was performed byemploying a −ΔV method in which the secondary battery was charged withthe charging current of 1660 mA until the charged capacity was reachedto 40% of the nominal capacity, followed by charging under the currentof 830 mA, and the charging was finished at the time when the voltagewas lowered by 4 mV from the maximum voltage in the charging process. Onthe other hand, the discharge process was performed under a current of1,660 mA until the battery voltage was lowered to 1.0V.

Also, characteristics (A) to (D) given below were measured in respect ofthe hydrogen absorbing alloy used in the secondary battery for each ofExamples 81 to 96 and Comparative Examples 21 to 25:

(A) A rechargeable hydrogen storage capacity (which is a term for ahydrogen absorbing alloy specified in JIS H7003) was obtained as ahydrogen absorption characteristic for each of the hydrogen absorbingalloys by measuring the iso-thermal line of pressure-composition at 45°C. and under a hydrogen pressure lower than 10 atm. by the Sievert'smethod (JIS H7201). Table 17 shows the results.

(B) The crystal structure of each of the hydrogen absorbing alloys wasobserved from the X-ray diffraction pattern by using a Cu—Kα ray as theX-ray source so as to determine the crystal structure of the principalphase. Table 17 shows the results.

In the hydrogen absorbing alloy for Example 93, the peak having thehighest intensity appeared at the value of 2θ falling within a range of42.1°±1° in the X-ray diffraction pattern, and a peak appeared at thevalue of 2θ falling within a range of from 31° to 34° was found as beingsplit into two. The intensity of the two-split peak was lower than thatof peak P1 in diffraction pattern (a) of the normal structure referredto previously. On the other hand, in the hydrogen absorbing alloy forExample 94, the peak having the highest intensity appeared at the valueof 2θ falling within a range of 42.1°±1° in the X-ray diffractionpattern, and a peak having an intensity lower than that of peak P1referred previously appeared at the value of 2θ falling within a rangeof from 31° to 34°. Incidentally, the intensity ratio calculated byformula (I) referred to previously on the basis of the diffractionpattern of the hydrogen absorbing alloy for Example 94 was found to be22%.

Further, the electron diffraction analysis pattern for the hydrogenabsorbing alloy for each of Examples 93 and 94 was photographed by atransmission electron microscope (TEM). It has been found that therewere four supper lattice reflection spots between the primitive latticereflection points (00L) and the origin (000) in the hydrogen absorbingalloy for each of Examples 93 and 94. It has also been found that thesupper lattice reflection spots were present in four points at which thedistance |G_(00L)| between the primitive lattice reflection spot (00L)and the origin (000) was equally divided into five sections.

Also, the peak having the highest intensity appeared at the value of 2θfalling within a range of 42.1°±1° in the X-ray diffraction pattern ofthe hydrogen absorbing alloy for Example 95. Further, a peak appeared atthe value of 2θ falling within a range of from 31° to 34° was found asbeing split into three. The intensity of the three-split peak was foundto be lower than that of the peak appearing in the diffraction patternof the normal structure referred to previously. On the other hand, thepeak having the highest intensity appeared at the value of 28 fallingwithin a range of 42.1°±1° in the X-ray diffraction pattern of thehydrogen absorbing alloy for Example 96. Also, a peak having anintensity lower than that of peak of the normal structure referred topreviously appeared at the value of 2θ falling within a range of from31° to 34°. Incidentally, the intensity ratio calculated by formula (I)referred to previously on the basis of the diffraction pattern of thehydrogen absorbing alloy for Example 96 was found to be 25%.

(C) A secondary electron image and a reflected electron image of each ofthe hydrogen absorbing alloys was photographed by using a scanningelectron microscope (SEM) so as to detect a phase differing from theprincipal phase. The composition of the phase differing from theprincipal phase was analyzed by an energy dispersive X-ray spectroscopy(EDX) of the scanning electron microscope. It has been found from theresult of the composition analysis and the X-ray diffraction pattern byusing a Cu—Kα ray as the X-ray source that the phase differing from theprincipal phase was a phase having a MgCu₂ type crystal structure.

(D) The content of the MgCu₂ type phase was measured as follows for eachof the hydrogen absorbing alloys so as to obtain the results as shown inTable 17.

Optional five view fields of the hydrogen absorbing alloy for each ofExamples 81 to 88, 93 to 96 and Comparative Examples 22, 24, 25 werephotographed by a scanning electron microscope. The area ratio of thetarget phase based on the alloy area within the view field was obtainedfor each of the micrographs. The average value of the area ratios thusobtained was calculated and given in Table 17 as the volume ratio of thetarget phase in the hydrogen absorbing alloy.

On the other hand, optional five view fields of the hydrogen absorbingalloy for each of Examples 89 to 92 and Comparative Examples 21, 23 werephotographed by a transmission electron microscope. The area ratio ofthe target phase based on the alloy area within the view field wasobtained for each of the micrographs. The average value of the arearatios thus obtained was calculated and given in Table 17 as the volumeratio of the target phase in the hydrogen absorbing alloy.

TABLE 16 Heat treatment conditions Temperature Time Composition (° C.)(h) Example 81 Lm(11)_(0.77)Mg_(0.23)Ni_(3.34)Al_(0.08) 935 9 Example 82Lm(9)_(0.61)Nd_(0.12)Mg_(0.27)Ni_(3.2)Mn_(0.02)Si_(0.01)Al_(0.07) 94010  Example 83La_(0.76)Ce_(0.02)Mg_(0.22)Ni_(3.43)Zn_(0.02)Al_(0.12)Li_(0.003) 930 6Example 84 Lm(10)_(0.76)Mg_(0.24)Ni_(3.34)Al_(0.11)Mo_(0.005) 940 5Example 85Lm(11)_(0.8)Mg_(0.2)Ni_(3.34)Co_(0.05)Mn_(0.02)Al_(0.11)Ta_(0.003) 915 7Example 86Lm(9)_(0.53)Pr_(0.2)Mg_(0.27)Ni_(3.1)Co_(0.1)Fe_(0.01)Al_(0.1) 945 6Example 87Lm(9)_(0.76)Y_(0.05)Mg_(0.19)Ni_(3.25)Co_(0.05)Sn_(0.01)Al_(0.09) 925 9Example 88 Lm(11)_(0.7)Mg_(0.3)Ni_(3.22)Cu_(0.03)Al_(0.08) 945 7 Example89 Lm(10)_(0.76)Mg_(0.24)Ni_(3.3)Co_(0.02)W_(0.002)B_(0.01)Al_(0.09) 9453 Example 90Lm(9)_(0.68)Mm(4)_(0.1)Mg_(0.22)Ni_(3.19)Co_(0.11)Al_(0.12)Ga_(0.03) 9302 Example 91Lm(9)_(0.67)Mm(4)_(0.1)Mg_(0.23)Ni_(3.3)Co_(0.07)Mn_(0.03)V_(0.002)Al_(0.08)935 1 Example 92Lm(12)_(0.76)Ca_(0.01)Mg_(0.23)Ni_(3.28)Cr_(0.002)Al_(0.12) 980 4Example 93 Lm(10)_(0.766)Zr_(0.004)Mg_(0.23)Ni_(3.31)P_(0.002)Al_(0.12)950 7 Example 94 Lm(11)_(0.77)Mg_(0.23)Ni_(3.16)Co_(0.11)Al_(0.12) 960 5Example 95Lm(9)_(0.72)Mm(4)_(0.06)Mg_(0.22)Ni_(3.13)Co_(0.2)Mn_(0.03)Sn_(0.02)Al_(0.09)945 8 Example 96Lm(11)_(0.775)Ti_(0.005)Mg_(0.22)Ni_(3.31)Al_(0.13)S_(0.002) 960 5ComparativeMm(4)_(0.65)Mg_(0.35)Ni_(2.27)Co_(0.3)Mn_(0.3)Fe_(0.02)Al_(0.14) 850 4Example 21 ComparativeMm(4)_(0.975)Mg_(0.025)Ni_(2.925)Mn_(0.35)Cu_(0.1)Nb_(0.075) 900 12 Example 22 Comparative Lm(10)_(0.917)Mg_(0.083)Ni_(4.75) — — Example 23Comparative Lm(10)_(0.34)Mg_(0.66)Ni_(3.2) 1000  6 Example 24Comparative Lm(11)_(0.5)Mg_(0.5)Ni_(2.2) 800 5 Example 25 Note: Thealloys for Examples 89 to 92 and Comparative Example 21 were prepared bya rapid solidification process, and the alloy for Comparative Example 23was prepared by a mechanical alloying method.

TABLE 17 Rechargeable hydrogen Content of storage AB₂ type CapacityCycle capacity Crystal structure of phase (% (mAh) life (H/M) principalphase by volume) Example 81 750 310 1.06 Ce₂Ni₇ type 0.4 Example 82 735305 1.04 Ce₂Ni₇ type + CeNi₃ type 0.5 Example 83 740 290 1.07 Ce₂Ni₇type 0.8 Example 84 728 295 1.05 Ce₂Ni₇ type + PuNi₃ type 1.5 Example 85753 280 1.06 Ce₂Ni₇ type 0 Example 86 745 285 1.06 Ce₂Ni₇ type + CeNi₃type 2.4 Example 87 738 275 0.93 Ce₂Ni₇ type 2.2 Example 88 730 280 1.06Ce₂Ni₇ type + PuNi₃ type 2.5 Example 89 755 265 0.98 Ce₂Ni₇ type 3.2Example 90 740 270 0.97 Ce₂Ni₇ type + Gd₂Co₇ type 4.5 Example 91 732 3050.92 Ce₂Ni₇ type 1.2 Example 92 739 250 0.87 Ce₂Ni₇ type 1.3 Example 93743 270 0.86 Similar to Ce₂Ni₇ type 1.5 Example 94 740 275 0.88 Similarto Ce₂Ni₇ type 3.5 Example 95 736 250 0.89 Similar to PuNi₃ type 4.3Example 96 742 220 0.85 Similar to PuNi₃ type 5.5 Comparative 680 800.66 CaCu₅ type 13 Example 21 Comparative 550 30 0.54 Ce₂Ni₇ type 17Example 22 Comparative 640 10 0.43 CaCu₅ type 0 Example 23 Comparative480 15 0.21 PuNi₃ type 21 Example 24 Comparative 450 10 0.35 MgCu₂ type75 Example 25

To reiterate, the hydrogen absorbing alloy for each of Examples 81 to 96had a composition represented by formula (4) given previously andcontained the AB₂ type phase such as the MgCu₂ type phase in an amountnot larger than 10% by volume. As apparent from Tables 16 and 17, therechargeable hydrogen storage capacity of the hydrogen absorbing alloyfor each of Examples 81 to 96 was larger than that of the hydrogenabsorbing alloy for each of Comparative Examples 21 to 25. Incidentally,the hydrogen absorbing alloy for Comparative Example 21 had acomposition equal to that of the hydrogen absorbing alloy disclosed inU.S. Pat. No. 5,840,166 and contained the AB₂ type phase in an amountexceeding 10% by volume. Also, the hydrogen absorbing alloy forComparative Example 22 had a composition equal to that of the hydrogenabsorbing alloy disclosed in Japanese Patent Disclosure No. 11-29832 andcontained the AB₂ type phase in an amount exceeding 10% by volume. Onthe other hand, the hydrogen absorbing alloy for Comparative Example 23had a composition equal to that of the hydrogen absorbing alloydisclosed in Japanese Patent Disclosure No. 10-1731.

It is also seen that the secondary battery for each of Examples 81 to 96was found to be superior to the secondary battery for each ofComparative Examples 21 to 25 in each of the discharge capacity and thecharge-discharge cycle life.

EXAMPLES 97 TO 112 AND COMPARATIVE EXAMPLES 26 TO 30

Hydrogen absorbing alloys were prepared by the high frequency inductionmelting method, the rapid solidification process or the mechanicalalloying method as described in the following.

(High Frequency Induction Melting Method)

Each of the elements constituting the composition shown in Table 18 wasweighed, followed by melting the composition in a high frequencyinduction furnace under an argon gas atmosphere so as to obtain an alloyingot. Then, a heat treatment was applied to the alloy ingot thusobtained under an argon gas atmosphere and under the conditions shown inTable 18 so as to obtain hydrogen absorbing alloys for Examples 97 to105, 109 to 112 and Comparative Examples 27, 29, 30.

(Rapid Solidification Process)

Each of the elements constituting the composition shown in Table 18 wasweighed, followed by melting the composition in a high frequencyinduction furnace under an argon gas atmosphere so as to obtain an alloyingot. The alloy ingot thus obtained was melted, followed by drippingthe resultant melt onto the surface of a single roll made of copper androtating at a peripheral velocity of 7 m/sec under an argon gasatmosphere so as to rapidly cool the melt and, thus, to obtain aflake-like hydrogen absorbing alloy. Further, a heat treatment wasapplied to the resultant hydrogen absorbing alloy flakes under an argongas atmosphere and under the conditions shown in Table 18 so as toobtain hydrogen absorbing alloys for Examples 106 to 108 and ComparativeExample 26.

(Mechanical Alloying Method)

A raw material metal mixture prepared by mixing Lm(11) and Ni at anatomic ratio Lm(11):Ni of 1:3 was melted in an arc furnace, followed bycooling the melt so as to obtain an alloy (a) consisting of a Lm(11)Ni₃phase. Also, another raw material metal mixture prepared by mixingLm(11) and Ni at an atomic ratio Lm(11):Ni of 1:4 was melted in an arcfurnace, followed by cooling the melt so as to obtain an alloy (b)consisting of a Lm(11)Ni₄ phase. The alloy (a) in an amount of 100 g andthe alloy (b) in an amount of 100 g were put in a planetary ball millhaving steel balls put therein so as to be subjected to a mechanicalalloying for 10 hours at room temperature under an argon gas atmosphere,thereby obtaining an alloy (c) consisting of a Lm(11)Ni₃ phase, aLm(11)Ni₄ phase and a Lm(11)₅Ni₁₉ phase.

On the other hand, a raw material metal mixture prepared by mixingLm(11), Mg and Ni at an atomic ratio Lm(11):Mg:Ni of 0.9:0.1:5 wasmelted in an arc furnace, followed by cooling the melt so as to obtainan alloy (d) consisting of a Lm(11)_(0.9)Mg_(0.1)Ni₅ phase. The alloy(d) in an amount of 30 g and the alloy (c) in an amount of 300 g wereput in a planetary ball mill having steel balls put therein so as to besubjected to a mechanical alloying for 10 hours at room temperatureunder an argon gas atmosphere, thereby obtaining an alloy forComparative Example 28 consisting of a Lm(11)Ni₃ phase, a Lm(11)Ni₄phase, a Lm(11)₅Ni₁₉ phase and a Lm(11)_(0.9)Mg_(0.1)Ni₅ phase.

A rectangular nickel hydrogen secondary battery was assembled as inExample 81 by using each of the hydrogen absorbing alloys thus prepared.

The secondary battery prepared in each of Examples 97 to 112 andComparative Examples 26 to 30 was left to stand at room temperature for72 hours. Then, the discharge capacity and the charge-discharge cyclelife were measured as in Example 81 for each of the secondary batteries.Table 19 shows the results.

Also, characteristics (A) to (F) given below were measured in respect ofthe hydrogen absorbing alloy used in the secondary battery for each ofExamples 97 to 112 and Comparative Examples 26 to 30:

(A) A rechargeable hydrogen storage capacity was obtained as in Example81 for each of the hydrogen absorbing alloys. Table 19 shows theresults.

(B) The crystal structure of each of the hydrogen absorbing alloys wasobserved from the X-ray diffraction pattern by using a Cu—Kα ray as theX-ray source so as to determine the crystal structure of the principalphase. Table 19 shows the results.

In the hydrogen absorbing alloy for Example 109, the peak having thehighest intensity appeared at the value of 2θ falling within a range of42.1°±1° in the X-ray diffraction pattern, and a peak appeared at thevalue of 2θ falling within a range of from 31° to 34° was found as beingsplit into two. The intensity of the two-split peak was lower than thatof peak P1 in diffraction pattern (a) of the normal structure referredto previously. On the other hand, in the hydrogen absorbing alloy forExample 110, the peak having the highest intensity appeared at the valueof 2θ falling within a range of 42.1°±1° in the X-ray diffractionpattern, and a peak having an intensity lower than that of peak P1referred previously appeared at the value of 2θ falling within a rangeof from 31° to 34°. Incidentally, the intensity ratio calculated byformula (I) referred to previously on the basis of the diffractionpattern of the hydrogen absorbing alloy for Example 110 was found to be20%.

Further, the electron diffraction analysis pattern for the hydrogenabsorbing alloy for each of Examples 109 and 110 was photographed by atransmission electron microscope (TEM). It has been found that therewere four supper lattice reflection spots between the primitive latticereflection points (00L) and the origin (000) in the hydrogen absorbingalloy for each of Examples 109 and 110. It has also been found that thesupper lattice reflection spots were present in four points at which thedistance |G_(00L)| between the primitive lattice reflection spot (00L)and the origin (000) was equally divided into five sections.

Also, the peak having the highest intensity appeared at the value of 2θfalling within a range of 42.1°±1° in the X-ray diffraction pattern ofthe hydrogen absorbing alloy for Example 111. Further, a peak appearedat the value of 2θ falling within a range of from 31° to 34° was foundas being split into four. The intensity of the four-divided peaks wasfound to be lower than that of the peak appearing in the diffractionpattern of the normal structure referred to previously. On the otherhand, the peak having the highest intensity appeared at the value of 2θfalling within a range of 42.1°±1° in the X-ray diffraction pattern ofthe hydrogen absorbing alloy for Example 112. Also, a peak having anintensity lower than that of peak of the normal structure referred topreviously appeared at the value of 2θ falling within a range of from31° to 34°. Incidentally, the intensity ratio calculated by formula (I)referred to previously on the basis of the diffraction pattern of thehydrogen absorbing alloy for Example 112 was found to be 19%.

(C) The content of the principal phase was measured as follows for eachof the hydrogen absorbing alloys so as to obtain the results as shown inTable 19.

Optional five view fields of the hydrogen absorbing alloy for each ofExamples 97 to 105, 109 to 112 and Comparative Examples 27, 29, 30 werephotographed by a scanning electron microscope. The area ratio of thetarget phase based on the alloy area within the view field was obtainedfor each of the micrographs. The average value of the area ratios thusobtained was calculated and given in Table 19 as the volume ratio of thetarget phase in the hydrogen absorbing alloy.

On the other hand, optional five view fields of the hydrogen absorbingalloy for each of Examples 106 to 108 and Comparative Examples 26, 28were photographed by a transmission electron microscope. The area ratioof the target phase based on the alloy area within the view field wasobtained for each of the micrographs. The average value of the arearatios thus obtained was calculated and given in Table 19 as the volumeratio of the target phase in the hydrogen absorbing alloy.

(D) The volume ratio of the parallel growth region in the crystal grainwas measured as in Example 17 for each of the hydrogen absorbing alloys.Table 19 shows the results.

(E) A ratio of the number of crystal grains, in which the volume ratioof the parallel growth region was not higher than 40%, to the totalnumber of crystal grains was calculated as in Example 17 and given inTable 19, said ratio being hereinafter referred to as a “crystal grainratio”.

(F) The composition of the parallel growth region excluding theprincipal phase of each of the hydrogen absorbing alloys was analyzed byan energy dispersive X-ray spectroscopy (EDX) of the scanning electronmicroscope. The crystal structure of the parallel growth was specifiedfrom the result of the composition analysis and the X-ray diffractionpattern obtained in item (B) described previously. The results are shownin Table 19.

TABLE 18 Heat treatment conditions Temperature Time Composition (° C.)(h) Example 97 Lm(10)_(0.77)Mg_(0.23)Ni_(3.28)Mn_(0.04)Al_(0.11) 930 5Example 98La_(0.62)Pr_(0.14)Mg_(0.24)Ni_(3.24)Mn_(0.02)Fe_(0.01)Al_(0.8) 955 9Example 99 La_(0.59)Nd_(0.16)Mg_(0.25)Ni_(3.1)Co_(0.1)Si_(0.01)Al_(0.09)975 11 Example 100 Lm(11)_(0.77)Mg_(0.23)Ni_(3.32)Al_(0.11)Ta_(0.002)945 6 Example 101Lm(10)_(0.8)Mg_(0.2)Ni_(3.34)Co_(0.05)Mn_(0.04)Al_(0.11)Mo_(0.005) 930 7Example 102La_(0.73)Ce_(0.05)Mg_(0.22)Ni_(3.33)Zn_(0.03)Al_(0.12)Li_(0.002) 920 8Example 103Lm(10)_(0.78)Y_(0.03)Mg_(0.19)Ni_(3.25)Co_(0.1)Sn_(0.01)Al_(0.08) 925 5Example 104 Lm(9)_(0.71)Mg_(0.29)Ni_(3.23)Cu_(0.02)Al_(0.1) 910 7Example 105Lm(11)_(0.76)Mg_(0.24)Ni_(3.31)Co_(0.03)W_(0.002)B_(0.04)Al_(0.08) 965 9Example 106 Lm(10)_(0.7)Mm(4)_(0.09)Mg_(0.21)Ni_(3.31)Al_(0.13)Ga_(0.02)955 2 Example 107Lm(9)_(0.72)Mm(4)_(0.04)Mg_(0.24)Ni_(3.24)Co_(0.06)Mn_(0.04)V_(0.002)Al_(0.1)950 1.5 Example 108Lm(12)_(0.76)Ca_(0.01)Mg_(0.23)Ni_(3.3)Cr_(0.003)Mn_(0.05)Al_(0.12) 9853 Example 109Lm(9)_(0.767)Zr_(0.003)Mg_(0.23)Ni_(3.28)Co_(0.04)P_(0.002)Al_(0.1) 96010 Example 110Lm(10)_(0.785)Ti_(0.005)Mg_(0.21)Ni_(3.29)Al_(0.15)S_(0.002) 970 8Example 111 Lm(10)_(0.76)Mg_(0.24)Ni_(3.18)Co_(0.1)Mn_(0.05)Al_(0.12)960 7 Example 112Lm(9)_(0.75)Mm(4)_(0.03)Mg_(0.22)Ni_(3.02)Co_(0.15)Mn_(0.01)Sn_(0.02)Al_(0.1)955 6 ComparativeMm(4)_(0.72)Mg_(0.28)Ni_(2.66)Co_(0.4)Mn_(0.4)Fe_(0.02)Al_(0.15) 900 4Example 26 ComparativeMm(4)_(0.975)Mg_(0.025)Ni₃Mn_(0.35)Cu_(0.1)Ga_(0.05) 900 12 Example 27Comparative Lm(11)_(0.917)Mg_(0.083)Ni_(4.75) — — Example 28 ComparativeLm(12)_(0.34)Mg_(0.66)Ni_(3.2) 1000  5 Example 29 ComparativeLm(10)_(0.5)Mg_(0.5)Ni_(2.3) 800 7 Example 30 Note: The alloys forExamples 106 to 108 and Comparative Example 26 were prepared by a rapidsolidification process, and the alloy for Comparative Example 28 wasprepared by a mechanical alloying method.

TABLE 19 Parallel Rechargeable Principal growth hydrogen Crystal phasecontent of Crystal Crystal storage structure of content crystal grainstructure of Capacity Cycle capacity principal (% by grain (% ratioparallel (mAh) life (H/M) phase volume) by volume) (%) growth Example 97745 305 1.03 Ce₂Ni₇ type 96 4 95 PuNi₃ type + A₅B₁₉ type Example 98 730300 1.02 Ce₂Ni₇ type + PuNi₃ 92 8 92 PuNi₃ type + A₅B₁₉ type typeExample 99 738 295 1.06 Ce2Ni₇ type 95 10 88 CeNi₃ type + A₅B₁₉ typeExample 100 726 290 1.05 Ce₂Ni₇ type + CeNi₃ 96 14 85 PuNi₃ type typeExample 101 752 285 0.98 Ce₂Ni₇ type 92 8 90 PuNi₃ type + A₅B₁₉ typeExample 102 746 280 1.03 Ce₂Ni₇ type + PuNi₃ 96 17 82 CeNi₃ type + A₅B₁₉type type Example 103 734 270 0.92 Ce₂Ni₇ type 92 18 83 PuNi₃ type +A₅B₁₉ type Example 104 732 275 1.01 Ce₂Ni₇ type + PuNi₃ 90 24 85 A₅B₁₉type type Example 105 752 270 0.98 Ce₂Ni₇ type 92 20 72 PuNi₃ type +A₅B₁₉ type Example 106 738 285 1.03 Ce₂Ni₇ type + Gd₂Co₇ 95 31 70 CeNi₃type + A₅B₁₉ type type Example 107 742 305 0.96 Ce₂Ni₇ type 94 5 90PuNi₃ type + A₅B₁₉ type Example 108 736 255 0.92 Ce₂Ni₇ type 90 11 88PuNi₃ type Example 109 748 280 0.9 Similar to 91 8 75 PuNi₃ type + A₅B₁₉Ce₂Ni₇ type type Example 110 742 285 0.86 Similar to 85 16 80 PuNi₃type + A₅B₁₉ Ce₂Ni₇ type type Example 111 738 260 0.88 Similar to 82 4065 Ce₂Ni₇ type + A₅B₁₉ PuNi₃ type type Example 112 740 240 0.84 Similarto 80 28 60 Ce₂Ni₇ type + A₅B₁₉ PuNi₃ type type Comparative 674 75 0.62CaCu₅ type 82 65 55 Ce₂Ni₇ type Example 26 Comparative 540 20 0.52Ce₂Ni₇ type 70 45 50 CaCu₅ type + PuNi₃ Example 27 type Comparative 63615 0.41 CaCu₅ type 90 5 88 Ce₂Ni₇ type + A₅B₁₉ Example 28 typeComparative 430 20 0.22 PuNi₃ type 88 15 78 Ce₂Ni₇ type + A₅B₁₉ Example29 type Comparative 426 15 0.3 MgCu₂ type 75 30 45 PuNi₃ type Example 30

To reiterate, the hydrogen absorbing alloy for each of Examples 97 to112 had a composition represented by formula (4) given previously. Also,the parallel growth region that has a crystal structure differing fromthe crystal structure of the principal phase precipitates in at leastone crystal grain of the principal phase. As apparent from Tables 18 and19, the rechargeable hydrogen storage capacity of the hydrogen absorbingalloy for each of Examples 97 to 112 was larger than that of thehydrogen absorbing alloy for each of Comparative Examples 26 to 30.Incidentally, the hydrogen absorbing alloy for Comparative Example 26had a composition equal to that of the hydrogen absorbing alloydisclosed in U.S. Pat. No. 5,840,166 and contained the CaCu₅ type phaseas the principal phase. Also, the hydrogen absorbing alloy forComparative Example 27 had a composition equal to that of the hydrogenabsorbing alloy disclosed in Japanese Patent Disclosure No. 11-29832 andcontained the Ce₂Ni₇ type phase as the principal phase. On the otherhand, the hydrogen absorbing alloy for Comparative Example 28 had acomposition equal to that of the hydrogen absorbing alloy disclosed inJapanese Patent Disclosure No. 10-1731.

It is also seen that the secondary battery for each of Examples 97 to112 was found to be superior to the secondary battery for each ofComparative Examples 26 to 30 in each of the discharge capacity and thecharge-discharge cycle life.

EXAMPLES 113 TO 120

Hydrogen absorbing alloys were prepared by the high frequency inductionmelting method as described in the following.

(High Frequency Induction Melting Method)

Each of the elements constituting the composition shown in Table 20 wasweighed, followed by melting the composition in a high frequencyinduction furnace under an argon gas atmosphere so as to obtain an alloyingot. Then, a heat treatment was applied to the alloy ingot thusobtained under an argon gas atmosphere and under the conditions shown inTable 20 so as to obtain hydrogen absorbing alloys for Examples 113 to120.

A nickel rectangular hydrogen secondary battery was assembled as inExample 81 by using each of the hydrogen absorbing alloys thus prepared.

The secondary battery prepared in each of Examples 113 to 120 was leftto stand under room temperature for 72 hours. Then, each of thedischarge capacity and the charge-discharge cycle life of the secondarybattery were measured as in Example 81 so as to obtain the results shownin Table 20.

The rechargeable hydrogen storage capacity, the crystal structure andcontent of the principal phase, the content of the AB₂ type phase, thecontent of the parallel growth region in the crystal grain, the crystalgrain ratio, and the crystal structure of the parallel growth regionwere measured as in Examples 81 and 97 in respect of the hydrogenabsorbing alloy used in the secondary battery for each of Examples 113to 120. Tables 20 and 21 show the results.

TABLE 20 Heat Rechargeable treatment hydrogen conditions storageTemperature Time Capacity Cycle capacity Composition (° C.) (h) (mAh)life (H/M) Example 113 Lm(9)_(0.76)Mg_(0.24)Ni_(3.28)Al_(0.12) 910 7 752330 1.05 Example 114Lm(10)_(0.77)Mg_(0.23)Ni_(3.24)Co_(0.05)Mn_(0.04)Al_(0.1) 925 6 738 3401.04 Example 115 Lm(11)_(0.77)Mg_(0.23)Ni_(3.24)Mn_(0.04)Al_(0.11) 95010 742 335 1.03 Example 116Lm(11)_(0.79)Mg_(0.21)Ni_(3.3)Mn_(0.06)Al_(0.1) 955 6 736 325 1.04Example 117 Lm(12)_(0.78)Mg_(0.22)Ni_(3.15)Co_(0.11)Mn_(0.03)Al_(0.11)975 12 748 305 1.03 Example 118Lm(11)_(0.77)Mg_(0.23)Ni_(3.24)Cu_(0.02)Al_(0.09) 950 8 742 320 1.06Example 119 Lm(10)_(0.77)Mg_(0.23)Ni_(3.28)Al_(0.12) 935 6 752 335 1.05Example 120 Lm(11)_(0.76)Mg_(0.24)Ni_(3.31)Al_(0.11) 950 5 746 340 1.04

TABLE 21 Principal Parallel Crystal phase Content of growth contentCrystal Crystal structure of content AB₂ type of crystal grain structureof principal (% by phase (% grain ratio parallel phase volume) byvolume) (% by volume) (%) growth Example 113 Ce₂Ni₇ type 92 0.9 6 92PuNi₃ type + A₅B₁₉ type Example 114 Ce₂Ni₇ type 95 0.4 7 90 CeNi₃ type +A₅B₁₉ type Example 115 Ce₂Ni₇ type + PuNi₃ 96 0.7 11 90 PuNi₃ type +A₅B₁₉ type type Example 116 Ce₂Ni₇ type 98 1.5 8 92 PuNi₃ type + A₅B₁₉type Example 117 Ce₂Ni₇ type 94 0.8 7 88 CeNi₃ type + A₅B₁₉ type Example118 Ce₂Ni₇ type + PuNi₃ 97 0.5 12 93 PuNi₃ type + A₅B₁₉ type typeExample 119 Ce₂Ni₇ type 93 1.1 8 95 PuNi₃ type + A₅B₁₉ type Example 120Ce₂Ni₇ type 96 0 5 95 CeNi₃ type + A₅B₁₉ type

As apparent from Tables 20 and 21, the secondary battery for each ofExamples 113 to 120 has a large capacity and a long charge-dischargecycle life.

As described above in detail, the present invention provides a hydrogenabsorbing alloy which permits increasing the hydrogenabsorption-desorption amount and also permits maintaining the increasedhydrogen absorption-desorption amount over a long period of time.

The present invention also provides a secondary battery having a largecapacity and a long cycle life.

Further, the present invention provides a hybrid car and an electricautomobile excellent in the running performance such as a fuel cost.

Additional advantages and modifications will readily occur to thoseskilled in the art. Therefore, the invention in its broader aspects isnot limited to the specific details and representative embodiments shownand described herein. Accordingly, various modifications may be madewithout departing from the spirit or scope of the general inventiveconcept as defined by the appended claims and their equivalents.

1. A hydrogen absorbing alloy, comprising: a composition represented bygeneral formula (3) given below and contains not higher than 10% byvolume including 0% by volume of a phase having an AB₂ type crystalstructure, and an intensity ratio calculated by formula (2) given belowbeing lower than 0.15 including 0:I₁/I₂  (2) where I₂ is an intensity of a highest peak in a X-raydiffraction pattern using a CuKα ray, and I₁ is an intensity of ahighest peak appearing at a value of 2θ falling within a range of from8° to 13° in the X-ray diffraction pattern, θ being a Bragg angle;R_(1-a-b)Mg_(a)T_(b)Ni_(Z-X)M3_(X)  (3) where R is at least one elementselected from rare earth elements, said rare earth elements including Y,T is at least one element selected from the group consisting of Ca, Ti,Zr and Hf, M3 is at least one element selected from the group consistingof Co, Mn, Fe, Al, Ga, Zn, Sn, Cu, Si, B, Nb, W, Mo, V, Cr, Ta, Li, Pand S, the atomic ratios of a, b, X and z are respectively satisfyconditions of: 0.15≦a≦0.37, 0≦b≦0.1, 0.53≦(1−a−b) ≦0.85, 0≦X≦2 and3≦Z≦4.2, wherein a parallel growth region precipitates in at least onecrystal grain of a principal phase, and the parallel growth region has acrystal structure differing from a crystal structure of the principalphase.
 2. A hydrogen absorbing alloy comprising a compositionrepresented by general formula (3) given below, wherein a parallelgrowth region precipitates in at least one crystal grain of a principalphase, the parallel growth region having a crystal structure differingfrom a crystal structure of the principal phase, and an intensity ratiocalculated by formula (2) given below is lower than 0.15 including 0:I₁/I₂  (2) where I₂ is an intensity of a highest peak in a X-raydiffraction pattern using a CuKα ray, and I₁ is an intensity of ahighest peak appearing at a value of 2θ falling within a range of from8° to 13° in the X-ray diffraction pattern, θ being a Bragg angle;R_(1-a-b)Mg_(a)T_(b)Ni_(Z-X)M3_(X)  (3) where R is at least one elementselected from rare earth elements, said rare earth elements including Y,T is at least one element selected from the group consisting of Ca, Ti,Zr and Hf, M3 is at least one element selected from the group consistingof Co, Mn, Fe, Al, Ga, Zn, Sn, Cu, Si, B, Nb, W, Mo, V, Cr, Ta, Li, Pand S, the atomic ratios of a, b, X and z are respectively satisfyconditions of: 0.15≦a≦0.37, 0≦b≦0.1, 0.53≦(1−a−b) ≦0.85, 0≦X≦2 and3≦Z≦4.2.
 3. The hydrogen absorbing alloy according to claim 2, whereinthe volume ratio of the parallel growth region of the at least onecrystal grain is not higher than 40% by volume.
 4. The hydrogenabsorbing alloy according to claim 3, wherein a ratio of the number ofcrystal grains, in which the volume ratio of the parallel growth regionis not higher than 40% by volume, to the total number of crystal grainsis not lower than 60%.
 5. A secondary battery, comprising a positiveelectrode, a negative electrode containing a hydrogen absorbing alloy,and an alkaline electrolyte, wherein said hydrogen absorbing alloy has acomposition represented by general formula (3) given below, a parallelgrowth region precipitates in at least one crystal grain of a principalphase, the parallel growth region having a crystal structure differingfrom a crystal structure of the principal phase, and an intensity ratiocalculated by formula (2) given is being lower than 0.15 including 0:I₁/I₂  (2) where I₂ is an intensity of a highest peak in a X-raydiffraction pattern using a CuKα ray, and I₁ is an intensity of ahighest peak appearing at a value of 2θ falling within a range of from8° to 13° in the X-ray diffraction pattern, θ being a Bragg angle;R_(1-a-b)Mg_(a)T_(b)Ni_(Z-X)M3_(X)  (3) where R is at least one elementselected from rare earth elements, said rare earth elements including Y,T is at least one element selected from the group consisting of Ca, Ti,Zr and Hf, M3 is at least one element selected from the group consistingof Co, Mn, Fe, Al, Ga, Zn, Sn, Cu, Si, B, Nb, W, Mo, V, Cr, Ta, Li, Pand S, the atomic ratios of a, b, X and z are respectively satisfyconditions of: 0.15≦a≦0.37, 0≦b≦0.1, 0.53≦(1−a−b) ≦0.85, 0≦X≦2 and3≦Z≦4.2.
 6. The hydrogen absorbing alloy according to claim 1, whereinsaid R comprises Ce in a content of lower than 20% weight.
 7. Thehydrogen absorbing alloy according to claim 1, which is manufactured bya sintering method.
 8. The hydrogen absorbing alloy according to claim1, which is manufactured by a high frequency induction melting method.9. The hydrogen absorbing alloy according to claim 1, which ismanufactured by a rapid solidification process.
 10. The hydrogenabsorbing alloy according to claim 1, which is applied a heat treatment.11. The hydrogen absorbing alloy according to claim 2, wherein said Rcomprises Ce in a content of lower than 20% weight.
 12. The hydrogenabsorbing alloy according to claim 2, which is manufactured by asintering method.
 13. The hydrogen absorbing alloy according to claim 2,which is manufactured by a high frequency induction melting method. 14.The hydrogen absorbing alloy according to claim 2, which is manufacturedby a rapid solidification process.
 15. The hydrogen absorbing alloyaccording to claim 2, which is applied a heat treatment.
 16. A secondarybattery, comprising: a negative electrode containing a hydrogenabsorbing alloy according to claim 1; a positive electrode; and analkaline electrolyte.